A better idea is to write a story yourself. That way you get exactly what you want.
And if you're having a hard time coming up with ideas, there are always other stories that may contain just a paragraph or two of an idea, a story line or a mood or feeling that works for you. Take that and then start embroidering your own story from there on ... The first medfet story I ever wrote started like that, by enlarging and adding on to a short story I once found that rang my bell ...
I really wouldn't pay for a story. Apologies to writers who may be trying to make a living, but they can never ever come up with anything remotely like what you are looking for. To write good porn, it has to rock your boat, otherwise it's lifeless ...
Back on Speculumpages, we received more than 3000 amateur stories, all sent in without any expectation of payment, purely for the fun and joy of writing them and sharing ... many of those stories are here in the archives. There ought to be something amongst them that can kick-start an idea for writing on your own ...
Back in the 1940s writers like Henry Miller and Anais Nin wrote sex stories for (wealthy) American collectors at $1.00 a page. But that only works if you meet face to face to hand over the story and get paid at the same time. And besides, what are the chances of finding a down and out writer as adept at writing as Henry Miller? .... ha ha