I have seen lot of escorts advertising couple friendly. These are mostly Caucasians.
Depending on how often you and your partner wants to do this you could also try the Seeking Arrangement. This will take more effort but could be rewarding if you find a nice bi girl. I have been using the site for the last 7 months and have had lots of fun so far. I have met a girl in Gold Coast who is bi and moving to Sydney in Jan.