no need to go through an agency,just visit some bars maybe in soi tiger have a look around make friends with some girls then ask the question.Be careful some girls arent true bi sexual so can be disapointing just be clear in what you want.
a little more advice make sure the hotel is guest friendly (no joiner fee )
try this link
http://www.phuket-info.com/forums/showthread.php?t=6567
some hotels only allow 1 girl without joiner fee
some dont care