There are many.
Depends on how much you want to spend.
Do you want to do it privately, if yes, can get a bisexual escort, or even a bisexual gogo dancer.
Go into the gogos with your gf, the bisexual girls will notes and head your way.
If your gf starts to play with them and they allow, you will know soon enough.
Try Kink, try Windmill, even bigger ones, Skyfall, XS, etc will have girls.
Beer bars and Soi 6 will be much harder to find.
The other option might be Gentlemens Clubs, or even a targeted profile on ThaiFriendly by your gf asking for women...