Call EZ-Roll Caster. I have bought from them a few times and they are great to deal with.
You want:
2 Double Locking (Swivel locking) casters in steel or poly-on-steel (hitting the brake locks both the wheel and the swivel)
2 rigid casters with brakes
I got 1250 lb capacity EACH for like $13/caster.
I called like 20 caster companies and distributors when I needed a set for a 1000lb band saw. They were by far the cheapest. Casters held up fine (that was nearly 10 years ago), although if you have a lot of rust problems, then you may want to step up to the USA-made (10-15x cost) ones.
Don't make the mistake I did on the first base I built and spec the casters for 1/4 of the weight of the machine like they tell you to do -- ended up with a wobbly frightening base. Now any caster can take the whole machine, and it's a lot more stable!
I didn't bother with leveling at all -- when your machine weighs 1000lbs and the casters are on plates that are sticking out from the machine, the plate will flex the 1/8" or whatever you need to make them all level (my plates are 5/8" thick medium steel 8" wide stock, attached with 7/16" pieces of 4x4 angle iron I think -- still get that 1/8" flex no problem when you lower the machine onto the base). No need for complicated designs. The double-locking brakes lock things down great. If I'm fine with putting my fingers next to a saw blade when the saw is on double-locked casters, your table will be plenty steady too.
Last thing -- make sure that when the swiveling ones swivel underneath the machine, the footprint is still wide enough so things don't get tipsy. I see too many 'first time' mobile bases where no one thought about that, and things are only stable with the swiveling casters swiveled in one direction -- not good!