I think you'll do fine finding Tinkerbell pins, there was plenty of them available from the CM's this past April. The Cinderella castle pins may be a little harder to find, but I wasn't necessarily looking for them, so there may be some about.
I found that the pins to buy in the parks ranged from about $6.00 up to about $12-13 each, depending on the type of pin. There are some 'interactive' onces which will set you back a little more.
You've done the right thing picking some up from ebay though, great way to get started on the cheap. Plus, if you get to the parks, you can buy a starter pack, complete with lanyard and four pins, two to keep, two to trade, that way you're well on the way to getting pins that you want to keep.
Myself and DBF loved this in April, we're buying some on-line before we travel next time so we've got some to begin trading with.
Enjoy and have fun, the CM's are great with this and will always spare a few mins to show off their pins and trade
