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.