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www.infinitystamps.com and www.thunderbirdsupply.com carry the stamps and can make custom stamps..
Another source for stamps is Harper (www.harpermfg.com)
Microstamp www.microstampusa.com
Timewell www.timewellcorp.com
Charm Factory http://charmfactory.com/detail.aspx?ID=2882
www.rings-things.com has the tags (chain tabs) in sterling and basemetal. (That's my business.) We can have your logo engraved or cut by a laser onto the small tages. There are minimum set up charges and a minimum quantity involved. Call us at 1-800-366-2156 Monday through Friday for details.
An internet search with the keywords "silver jewelry tags" will get you started with these and other vendors.