Don't be fooled by people who claim to have autographed balls from Sergio Garcia.  I actually know Sergio and know that he does not sign balls (like most Tour pros who want to avoid having permanent marker all over their hands).  You will never see any authentic signed balls from Sergio on eBay...try finding a picture of Sergio signing a ball if you don't believe me.  Ask Sergio to sign a ball in person and he will politely tell you that he does not sign balls and offer to sign something else for you.

If you want a real Sergio autograph I suggest attending a practice round to a PGA Tour event where he is playing.  Sergio spends a great deal of time with fans on Monday, Tuesday and Wednesday preceding his tournaments.  During this time he will sign items such as hats, pin flags, cards, and pictures in between holes and after his practice round.  If you're nice and polite, he will also take a picture with you.  He also often signs after his tournament rounds Thursday-Sunday, but since the PGA Tour's rules forbid him and other players to sign during the actual round you have a better chance at the beginning of the week.

Since he does sign a lot of autographs there is a great supply of authentic autographs on eBay.  However, consider the motive behind these products.  I have seen the memorabilia dealers before at the tournaments.  The camp out with bags full of merchandise to have signed by different athletes.  Sometimes they'll pay small children to get the autograph for them and other times they'll bud in front of children who actually admire Sergio.  If you are a true Sergio fan you will think twice before supporting this.  The memorabilia dealers don't actually care about Sergio they just want to make money off of you.

If you desperately want a Sergio autograph and you cannot make it to one of his tournaments I suggest buying an Upper Deck certified product because Sergio has an official contract with the company to sign cards for them.  These cards also make a good reference for comparing Sergio's authentic signature to another that you are considering.  However, realize that Sergio's signature looks very simple and could easily be duplicated by hand.