I have actually seen packages of random stamps you can buy at Michaels, but that was like a year ago. I remember they came in a red cloth bag and you got 300 for $2.99, or something. So I would recommend checking your local craft store, like A.C. Moore, for bunches of stamps. The only reason I had so many different ones was because my dad works for a company that does business with clients all over the world. He saved the stamps from back in the day when email wasn't used as much yet.
LOL, the old snail mail days. I am fortunate to have to pass both Michael's and A C Moore on my way to and from work so, I will check there. Were they in with the scrap booking stuff or the kids section?