eBay has become the best place to buy the vintage nostalgic aluminum Christmas trees.  It's best to buy them in early spring to get the best deals.  However you can still get really great deals all year long.  To make sure you have a great looking tree, make sure they still come with the sleeves for the branches.  Those that have been stored without the branches tend to break apart, or the aluminum is coming away from the steel branch.  It's best to find one with the original box as well.

POM POM tips are the most popular.  They were also the premium trees when first on the market.  Promotional (less expensive) trees did not have pom pom style tips. 

Check for how many branches come with the tree.  The more branches the better the look. The most important features for look are   

1. number of branches, under 75 was a low priced promotional tree, over 150 were the more deluxe trees
   
2. length of the branches, typically 12" to 24" long
   
3. pom pom or tip style
4. how tall is the tree
5. maker or manufacturer, Evergleam, Sparkler and Peco were some of the most popular brand
   
6. does it come with sleeves and the box
7. color of the aluminum, silver was the most popular but it was also produced in pink, green, gold, and blue
   
8. does it have the original stand

Pink trees are the most expensive of all, very rare, and hard to find.  You may have to wait awhile to find a rare sytle tree, but sooner or later it will appear on eBay.  Set up a search and have eBay email you whenever a new item appears for your search criteria.