Hi
Why do you want used ones, may i ask?
Ebay is the best place because most people 'in the trade' hang onto their stock of numatic dry-vacs as the parts are interchangable with other numatic models.
The used numatics on ebay go for stupidly high prices. You can get a Henry for £80 new in some shops and Halfords were doing their own-brand version of the Numatic Nuvac (the nuvac being a watered-down Henry, but just as reliable), someone kindly posted a message about it in the general forum, i'll have a look for the link.
Stephen