Quite a few of the single women or women in groups rent gigolos. Phuket gigolos are the equivalent of "Kuta Cowboys" that single women seek out in Bali. (Many of these women come from Australia.) But I never read a lot about lesbians seeking lesbian bars. You can check with a ladyboy bar, perhaps they will know.
I have no personal experience with lesbian or gay bars - anywhere - but you will have some sources using Google. Like this:
http://www.phuketlesbianbar.com/
http://www.thaivisa.com/forum/topic/320209-lesbian-club-in-phuket/
If all else fails contact some of the members on this expat forum below.
http://www.thaivisa.com/forum/forum/27-gay-people-in-thailand/
http://www.thaivisa.com/forum/forum/15-ladies-in-thailand/
http://www.thaivisa.com/forum/forum/31-phuket-forum/
You have nothing to lose by visiting a beer bar and asking a bar lady to go with you. Money is money and this is off season!