I have had good luck with, Hot Rod Hot Line, and Northwest Classic Auto Mall.com. Lot's of selection, always changing.
https://www.allcollectorcars.com/dealer/Streetside-Classics-Phoenix/1494/ I have used this one and great to see what they actually sell for with locations other than Phoenix.
Check out RMSothbys and Bonhams auctions. There are frequently "fish out of the water" customs for sale. You can go to "results" to see prices similar cars brought in previous sales. "No reserve" cars are of particular interest because the bidding pool is very limited and you have to be a registered bidder to participate.