Welcome and good luck!
Another source for GTO's to purchase would be to join the GTOAA (GTO Association of America) --- just to get their monthly newsletter. In the back of each issue every month is a classified ad section and there are always GTO's for sale. Yes, they tend to be pricey considering that these will be enthusiast/member cars, there are always some available. In my current issue alone there are two 68's listed for sale. One Midnight Blue, YS engine, Dual Gate auto, PHS documented listed at $28,000. Another Nightshade Green, dual gate, loaded, "national class winner", 3rd owner since new, PHS documented, listed at $53,500
There are also "Pontiacs for sale", "Pontiacs wanted", "Parts for sale", "Parts wanted", and "Services" sections in the newsletter available to members. Good resources...