Ebo I'd just check out premium aquatics and/or inland aquatics since you are so close. probably get a good deal.
But otherwise GL has a good suggestion.
I have bad luck mixing snails and crabs (the latter eat the former and I prefer the former, oh well). Coupla rules: the ONLY nassarius you want are nassarius VIBEX. Don't believe what anyone says about the various other types you can find on ebay especially. Second, NO bumblebee snails ... they are carnivores, will destroy your sandbed by devouring your pods and worms.
If you do a search on "detritivore" on ebay you will find Rob Sullivan. I recommend his kits highly, and you can ask him to sell you extra snails.
There is also a guy who sells the nassarius vibex at very good prices. Email me and I'll send the link.