Threesome, but I'm not comfortable.

Ok, so how do I look for a threesome partner with my BF? He wants to have one and wants me to find the girl but I get nervous and don't wanna look freaky as hell asking random girls. I'm not even gay or bi; however I can find a girl attractive. I have always been with a guy so how can I feel comfortable doing sexual activity with a girl. I think it could be fun after the 3rd time or so but I always been private wit my sexual life. So this is very new. Tell me what you think.