My favorite that I've heard is called "Echoes of Nature Thunderstorm". Amazon doesn't sell the MP3 of the main track, but you can pick up a
used CD for cheap. My wife and I call it "sleep storm". We like to listen to it while we nap. It usually puts us both into a coma of profound napping. You can rip it as an MP3 or FLAC or whatever format you wish using EAC or CueTools or whatever CD ripper software you prefer.
It's not just thunder, though. It's mostly rain. If you have something different in mind, or if you were looking for something for free, then grab the Video DownloadHelper add-on for Firefox and use it to rip something from YouTube. Then use any of a thousand free programs to rip the audio track from your downloaded MP4 as an MP3.