FPGAs aren't much good for mining Bitcoin, or even most altcoins, anymore - they were only briefly interesting as a gap between cpu/gpu and ASICs). The EEVblog forum is a fairly popular destination. I'm sure reddit has a few subs as well.
This forum, of course, also has a marketplace:
https://bitcointalk.org/index.php?board=51.0Just be sure to note that you're not selling these *as* miners, or you'll just get a bunch or sarcastic replies