These are the two main clubs:
http://www.gutshot.com/bforum
http://www.thewesternclub.com
But recent changes in how they're run to comply with not being licensed now mean you need to sign up 48 hours before you can play and you need to be proposed by an existing member:
http://gutshot.com/bforum/showthread.php?t=24024
Your best bet in light of this is the Grosvenor Victoria ("the Vic") which has a casino license so will let you turn up and play straight away.