As for the sound, no matter what you find I believe you'll have to edit it to make it a 10 minute (or whatever) long file. While these MP3 players will repeat a single stored MP3 song, there's a small gap. So what I do for repetitive sounds that might need to go on for minutes is to use a sound editor like Audacity and cut-and-paste a chunk of sound over and over until I build up a 10 minute song. Of course this still takes only a trivial amount of memory easily fitting into any microSD card.
Point being, if you're going to edit anyway it doesn't matter that it's an MP3 file to begin with - kind of sideways thinking on my part but that's my opinion. The first Google result from "railroad crossing bell sound" was this video which sounds pretty good to me. There's probably a converter programs to convert youtube audio to MP3 - what I know how to do is capture the sound within a sound editor, find convenient splice points, cut-and-paste to make the 10 minute file, then "save-as" MP3. Kind of old-school but that's what works for me...
