i usually go to my lfs and just ask for some of the nuisance snails (you know, like that come with plants) and they typically are more than happy to give them to me for free. they look at me funny but give them to me anyway. then i put them into my various tanks to let them reproduce. i have puffers too, thats why i get them. won't take them long to reproduce either. petsmart/petco stores are usually quite happy to have you help with their snail problem. some private stores might try to charge you (one was willing to give me a dozen for a dollar, theyre even selling ramshorns for $1.50, i got them free at petsmart). call around and see.