It would depend on how long you wanted to run the devices. Each workstation/printer pair could be connected to just about any 240v 8kAH UPS (they run about $10k) and it would keep those going for 10 to 30 minutes (much more when not printing). You could add extension batteries to double that time (or more); each extension is going to cost though.
We use the APC BACK-UPS series - both 550 and 700 rated versions. These are mainly to allow for small power glitches i.e. "brownouts" so the PCs keep going or to give us a few minutes to shutdown in the event of a "proper" power cut. They don't give much run-time.
Note that just because a workstation has a 500W power supply, it won't need as much as that in terms of UPS power - your best bet is to buy a small monitor that you plug into the power supply then plug your equipment into and it tells you how much power you are drawing and the load factor (generally > 90% for most IT equipment so not a worry) so you can then work out the UPS you need for the run time you want