I haven't seen any good "support beam" bricks available from Pick-a-Brick. You might be able to pick up a job lot from Bricklink or eBay, but that'd be kinda random.

There are some nice pillars with the Coast Guard Platform set, but it's kinda expensive for just the girders. You may need to really, really cheat by building a single pair of vertical supports and a roof truss out of however many bricks you have availalbe and then photograph it ten times each time further away to make it look like a long warehouse and then use the resulting image as a background image like Si665 said, or bluescreen it.