We use an Olec 5K. I picked it up end of summer from eBay, under for $400. I replaced a bulb and a relay on the PCB to get it going (several weeks of troubleshooting). New units and parts can be picked up from Douthitt. With a little bit of hunting and luck you might be able to find one on the used market. But it is a crapshoot out there. It is a very strong light source, coming from our 3000 watt Amergraph the quality is night and day. We burn two 23x31's on our vac frame. I don't have much else to compare it to other than our amergraph.... but with screens that big I honestly don't think I'd want anything less than 5K watts. I'd look at the Olec 5K or Nuarc 6K. If you're buying one new, expect to pay a lot of $$$. The 5K has been incredibly sweet to us so far. I think Satti has a single point LED on the market as well, but I wouldn't go LED unless I had no other choice.
