You want to find an animal that is already on F/T and a consist ant feeder at that.
Getting something that eats live might prove itself to be difficult to switch by yourself and I tend to always recommend to feed the same prey for at least 3 meals before switching to F/T otherwise you might end up with an animal that will just refuse food week after week, not to mention even after that switching can be as fast as one meal just like it can take a few months.
There are breeders that sell established animals feeding on F/T, some will switch them for you as well if it's the animal on a payment plan and has to stay a bit longer with the breeder, that being said taking one F/T does not mean the animal is switched, for example I only list or release animals as F/T feeders if they took at least 5 successful consecutive F/T (in my experience if they have taken less they might not truly be switched yet).
So when you buy your snake ask if the snake takes F/T, how many as he took and were those consecutive feedings (switching back and forth each week does not count)