Now, as for actually living this fantasy out. As I said before, this can get very complicated, very quickly. It can be something that is loads of fun to talk about, but very messy in real life. Some people choose to fulfill this fantasy with a prostitute to avoid the emotional entanglement. That is however, illegal and expensive and there is the risk of sexually transmitted diseases.
Some people prefer a friend because they feel more comfortable.
Of course, you then continue to see this person in social situations which could be awkward, this could increase the possibility of tension, and there is the risk of sexually transmitted diseases. Some people opt for something in between, and try to make a new friend to have these encounters with. Check the personal ads in your local paper. Usually, free or alternative papers will include ads for bi-curious or couples. You can also consider taking out your own ad.