Hello and welcome to the forum and to pin trading itself.
In terms of where to buy pins I would be careful on ebay because a lot of sellers, even those with good feedback, sell fakes and scrappers etc. Especially in the cases of pin lots. Its very tempting I know, but it would be so much worse to spend less money on fake pins you can't trade, than it would to just spend a little extra and get real pins.
In terms of looking for pins before you go, I would suggest you do a little searching on the forum - I can't remember the exact thread but I will go searching for it in a minute - but with a little searching you can find some really good deals on here, for single pins and booster sets.
The disney store is also a good place to get pins. You can usually pick up 'starter packs' that come with a lanyard and some pins or booster packs that come with a few pins and aren't too expensive.
I would recommend taking some pins with you to Disney, so that you can really get into trading as soon as you get there. But definitely pick up more pins whilst you're there, even if they are a little expensive, I'm sure you'll find some really nice ones, or ones that you could potentially trade whilst you're there.
The most important thing is to have fun with it though!