Is it necessary to buy the adapter from the original manufacturer?  If not what other choices do I have?

There are two choices available, buying an original adapter or a third party replacement.  The answer
to these two options is, price and quality.  Talking about the quality of an original and a
replacement adapter, Original adapter meets propriority standard.  For replacement adapters,
there could be two different standards.  The first one is no standard, the second one is, CE Certify.  

No standard : Some manufacturers offer product of no standard, they only make adapters to make money, if you have experience buying a third replacement before, I am sure you have encounter adapters that only work for a month or two...

CE Certify : Some manufacturers, however, produce adapters that means the CE Certify standard.  This is a standard employed in Europe, adapters meeting this standard is usually a better product, and should be able to last for a year, or two if used properly.

How to search for the right adapter? 

It goes by the laptop's series number. This number is usually found in front of the laptop panel.  Most of the time you only need that series number in order to find the right adapter to match your laptop.  However, if your laptop is made by Toshiba, it is required to get the full model number in addition to the series number.

Examples of some Toshiba laptop series that needs to go with the model
number are, Toshiba Satellite A35 series, P10 Series, P15 Series, P20 Series & P25

Example of series number plus model no. : P25-S477, P25-S487, P25-S507 and etc...

What to look for? What is CE Certify

The CE marking (also known as CE mark) is a mandatory conformity mark on many products placed on the single market in the European Economic Area (EEA). The CE marking does not certify that a product has met EU consumer safety, health or environmental requirements.

Significance

By affixing the CE marking, the manufacturer or person placing the product on the market or putting it into service asserts that the item meets all the essential requirements of the relevant European Directive(s). Examples of European Directives requiring CE marking include toy safety, machinery, low-voltage equipment, terminal equipment and EM compatibility. There are about 25 directives requiring CE marking.[1]
Officially, CE has no meaning as an abbreviation, but may have originally stood for European accordance (European Community) or Conformité Européenne ("European Conformity").

How do I make sure I get an adapter that last more than 6 months? 

If you pick the right supplier, plus you use it in the correct way, it will last for about 1-2 year.
When using it, make sure it has good ventilation. it is strongly suggested not to place it on a
carpet, or cover it with any type of fabric, blanket or clothes.  This is because when using the
adapter on less ventilated area, the dissipate heat may shorten the life of your adapter.  If it
gets too hot, it may melt the wire inside and stop working... by then you will need to get a new
one.  For normal usage under good ventilation, a good adapter will not get too hot and melt.
 

Today my laptop stop charging, should I shop for a new adapter immediately? 

No, you need to make sure the adapter is the problem, otherwise it is a waste of money.  These are the steps...
1. If you own more than one adapter, try your second adapter, if both are not working, the problem may be due to a malfunction mother board or battery.  If you only have one adapter, you may borrow one from your friend.
2. If the problem is due to a bad battery, you can remove the battery from the laptop, use only the adapter to supply power, if the adapter can get power supply without any problem, that means the battery has stop working, thus you need to get a new battery.

You just received your new adapter, but it is not working, so now what?
The 2 most common reasons for a malfunction new adapter... 
(1) the battery did not get charged, no power was detected;
(2) the adapater supply power to run the laptop, but the battery did not get charged

(1) Most adapters works right away, but there will be exceptions... chances are you may get a
lemon (defective/ live or dead adapter), or one that lives much shorter than normally
expected.  How to distinguish a defective adapter from another a good adapter?

1. Test your power cable.  The situation may be due to dead cable, please try with another cable just
to make sure.
2. Test your battery, remove it from the laptop, if the laptop can get power from the adapter when
the battery is removed, your battery is dead.

(2)The adapter doesn't supply enough power (voltage below specified) turn off the laptop, if the
adpter can only charge up the laptop when it is off, but no when it is on, that pretty much can tell you
it is under power.  You may need to return it to your supplier and get another one.  Chances are, you will get this problem resolved by changing to another one.  In my experience it is not commonly occur if the adapter has a pretty good quality offered by the manufacturer, so we shouldn't need to worry too much about this.

When I plug in the adapter, an error message displays saying that the voltage is not correct.

This problem can usually be fixed by switching to another adapter, due to individual failure of
a particular adapter.  Please contact us if you come across this situation.

A lot of sellers recommend getting an adapter of higher voltage, is it safe to get a higher
voltage adapter?

This is ok, to my best knowledge, for example, if you owns a Dell D620, D640, the original adapter
that comes with the laptop was 19.5v 3.34A, 65W, the compatible list of adapter 19.5V 4.62A,
90W include D620, D6420 and many others laptops

It is quite normal.  This is because we have check different adapters suggested by the different
laptop brands, like Toshiba is one of them, they suggest to get a replacement adapter of 120V
to different what laptop model and series that originally comes with lower voltage. 

As for Dell, they suggest some of their laptop models originally come with a 65W adapter
to get a 90W replacement.  In order to follow the manfacturer's suggestion, sometimes we
also suggest our buyer to go for an adapter with higher voltage thus higher power.