It takes lots of practice to be able to take a good size in pussy or ass , it's not something you can just turn up to a meeting and be able to fist some one.
As someone who enjoys being pegged , I can tell you it took many weeks starting with a finger, gradually moving up to rubber cock then after getting comfortable with that before going bigger. I can now take a good sized butt plug, but it takes a good hour to relax and open up to receive it. Being bi , I've been top and bottom with girls and guys, and can honestly say it is not a rush job. You will need the recipient to be used to taking something bigger than a cock and that with an SP is going to take a fair bit of time and obviously money. I'd suggest a civvie relationship and gradually work up to suggesting it , but I don't think you will easily find a sp who will let you fist her. You may find a guy more easily though.