I haven't been to this part of town for a bit, but on when you walk on the overhead pedestrain walkway from wanchai MTR to immigration tower, on the left (heading north) you can see into the window of a shop on the first floor that sells just what you are after. I haven't been into the actual shop for donkeys ages, but I think it is still there - they were renovating for sometime.
otherwise, Apliu st Sham Shui Po has a couple of shops that have some cheaper stuff in stock.
i think tin cheung in TST has a few strobes, and I'd wager they can get anything you want in the way of stands and backgrounds if you asked, but they don't have these in the showroom.