Mate,
That's a hard one - but I've been looking around - and I've decided that I'm going to use a pressure cooker (like you use on top of the stove to cook stews) for the pressure vessel. They're tough, and can hold a great deal of pressure, therefore can also take a great deal of negative pressure. You can drill a hole in the vessel for the fittings for the lines to the pump.
The vacuum pump is another matter - I've found a likely candidate at a local jewllery manufacturing supply store. They're not cheap - but it pays to spend a little more than you think you can afford - you'll only kick yerself later if you find that you should have bought the more expensive one.....
These chambers are used a lot for plaster - when someone is making a "lost wax" type of casting (done a lot with rings, brooches and bracelets), so you might try to check with these types of stores. They should have a fair number of different sizes to pick from.
Alternatively, check the yellow pages for "vacuum pumps" - and make sure that the pump is large enough to adequately evacuate the cubic capacity of the pressure vessel quickly enough - and make sure it can create vacuum to minus 0.9 bar (1 bar is about normal air pressure at sea level, or 101.3 kilopascals).
If you're evacuating air from RTV rubber molds, the time frame isn't really that important, but remember, if you're evacuating the air from polyurethane resin, you have to take into account the pot life of the resin - if it "goes off" (or hardens) before it can be fully evacuated, you'll just end up with a bubbly, foamy hardened mass. If you can evacuate it quickly enough, the resin will lose all air bubbles, and will fill all voids in the mold.
Good luck, mate.