As the other guys have suggested making your own is always good as you'll know it's unique! and really is pretty simple assuming you have a scanner. I did the exact same thing for a stationery design I did on a project back at Uni, I was working on the branding for a fictional fairtrade coffee company and added the coffee stains to the business cards and letterhead!
If you get really stuck let me know and I'll see if I can dig out my scans for that.
Good point Greg, I've still got a couple somewhere too so I'm happy to send them if you aint got a scanner (you could give Greg and me feedback out of ten to see who's the undisputed king of coffee stain making).