Almost everyone seems to use the Bitparking Namecoin Exchange.
bitparking is not recommended as they don't know what the word BACKUP is and is holding people coin now because their HD crashed and have not fixed the issue. I highly recommend you stay away from them.
There are so many other alt coin exchanges out there.
I am currently using:
Use the Bitdango market search to find Namecoin markets:
Then look at each market's order book, more orders means more liquidity (probably) so that's one way to base your decision.