At the moment there are three trustworthy exchanges for Namecoin with an acceptable price and a real trade volume:

livecoin.net

You pay: 0.00003739 BTC/NMC
You get: 0.00003510 BTC/NMC

southxchange.com

You pay: 0.00003715 BTC/NMC
You get: 0.00003471 BTC/NMC

altilly.com

You pay: 0.00003590 BTC/NMC
You get: 0.00003510 BTC/NMC

Warning: Beware of the Russian scam exchanges Yobit and BTCsquare! They will steal your coins and credentials, block your 2FA-login to demand money for unlocking! Beware further of the Chinese exchange CoinsEx which is giving the impression of having the highest trade volume - there are just shown fake trades!