Turning a fantasy into reality is something that not very many people get the chance to do. There are a lot of different types of fantasies out there. It is up to you on how you wish to make them a reality. Whatever your fantasy is, chances are you will be able to find a willing partner or partners that will be more than happy to help you turn it into a reality.
Where to Look
Depending on what your fantasy is, the best place to typically look into making it happen is online. Look up sites that are dedicated to whatever it is that interests you sexually. Most of these sites can provide to you message boards and forums as well as chat rooms filled with people that share your same interests. Always use caution whenever you meet with somebody on the internet.
Legal
No matter what it is that you are into, there are ways that you can make it something safe and legal. Even if you are into things that can be considered dangerous and therefore illegal, look into role playing. Role playing is something that can help to make any fantasy turn into a reality. You will simply need to find a partner that is willing to take part of this fantasy with you. It is a good idea to go with somebody that has experience in role playing so that they are more believable. Somebody that is not believable in role play can kill the mood.