I was in your shoes back in November. I looked here, rennlist, ebay, auto trader, craigslist, cars.com and dealer auctions. I looked at dozens of cars, PPI'd 2 and bought an 87 cabrio I found here.
I would say here and auto trader had the most results for me. But really, it's just a question of waiting for the right car to come along. 3-4 months is a fair bit of time, I'm surprised you haven't found something yet. High season is coming. That does mean higher prices, but also more selection.
If you're looking at cabrios, I looked very closely at this one:
1988 Porsche 911 For Sale - CarGurus
It's for sale by a dealer, but on consignment from a private owner whom I talked a lot with, got lots of pictures. It's a really nice car. I didn't get as far as a PPI, but it was a very strong candidate.