Rigging equipment for steaming large amounts at once
Assuming that you're planning to do the steaming at home (your question is kind of confusing that way), the cheapest way to go is to create ad-hoc "steam ovens" using disposable aluminum foil pans.


Buy some disposable aluminum foil baking pans, as large/deep as possible, like a 20" x 12" x 3.5" pan.
line the bottom of these with cookie cooling racks (if you don't have these, use an appropriately-sized disposable rimmed cookie sheet and punch holes in it).
pour in a little bit of water, below the tops of the racks.
put in the buns (should hold a lot of them in each pan)
seal tightly with extra-wide, heavy-duty foil on top.
bake in your oven at 350F


Now, the catch to this is that the cooking time will be different than it would n a regular steamer, because you're getting some heat from the top, and I don't have a ready conversion for you; you'd have to test a bit.

If this is something you do a lot, I'd recommend buying a Thai/Vietnamese-style multi-level steamer, which would allow you to steam batches of 12 to 30 buns pretty rapidly.  You can buy these steamers at Asian grocery stores a lot cheaper than you can find them online.