Cheap...well, of course that's relative..but the cheapest will probably be an Omega D series, preferably a D2V. Second-cheap, probably a Beseler 45MX type; the Beselers have been around a while, so I'd avoid the earlier kind that has the support struts in front of the main rails. It can be tough to fit an easel in there.

I recommend Saunders/LPL, Durst (parts can be difficult to find) or DeVere (parts can be impossible to find in the US).