http://www.autotraderclassics.com/find/vehicle/vehicleSearchResults.xhtml?conversationId=378505&firstRecord=1&numRecords=25&distance=0&address=&startYear=1968&endYear=1970&minPrice=20000&maxPrice=30000&make=Pontiac&model=Firebird&keywords=&sellerType=All&refined=true

use Auto trader classic

http://www.autotraderclassics.com/index.xhtml

http://www.autotraderclassics.com/find/index.xhtml?referral=semmsn&utm_source=MSN&utm_medium=ppc&utm_term=search+classic+cars&utm_campaign=ClassicCars&utm_content=SI_218657309_633312438_1

Just be aware that for that kind of money you could buy a really nice used low mileage late model Corvette that would retain it's value, be faster, get better mileage and be a lot easier to keep in good shape.