In my area its just as easy to find a Fox Body for sale as it is a C4 Vette (although finding a CLEAN fox is a different story).
In general, the C4 (84-91) is not a very popular car. I have yet to see anyone remotely in the same age range as me driving a C4. The aftermarket support is also very poor for the early C4s so it is not really popular among tuners as well. Pretty much the only people in the market for these cars are older retirees looking for a cheap fun car.
As for the fox, I think it is slowly becoming a classic and even though they are still relatively dirt cheap, the value of these cars bottomed out years ago and is SLOWLY increasing. This is pretty much due to the fact that Foxes had a very large impact among the public during the 80s and early 90s. Many people (including myself) remember these cars at an early age.
As for an 88 GT, I don't see anything about it that would make it harder to find than other year GTs unless you're talking about an '88 T-Top. If you're looking for a Fox I would suggest extending your search to the LA area. There are plenty of Foxes for sale here.