I always had good luck with Panasonic KX-P11xx series dot matrix printers. Back when you could still buy dot matrix paper and ribons these were the best printers because their were parts available everywhere and they were so common that folks would just give them away.
I also had good luck with HP ThinkJet dot matrix printers, but I don't know if these would work as well for your application.
As Deker said, there are many businesses that must have dot matrix printers, Okidata leads that market. They usually get good money for them, but the Okidata printers today are just as unstoppable as they were 20 years ago.
I also agree that ebay is the way to go, unless you have a university surplus action near you. You can often pick up equipment like that for fairly cheap, we have one near here at NC State, I've seen a lot of good stuff culled out of there over the years.