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I've never done it, but you may be able to do it via the 'Upload MOC' or 'Upload Inventory' at Bricklink / Rebrickable / BrickOwl. I think you can submit something and then 'rebrick' it, that is, get it to put together an order from the inventory list, with it automatically selecting vendors that have the parts. I have no idea if that's right, have just picked it up from on-line chatter I've come across, and I don't have a log-in for any of these sites so can't explain further. Perhaps someone else here has done it?
Buying from BrickLink - 2x2 plates in various colours. 2500 in total. Almost certainly one seller wont have them all. Any tips for buying? I assume this is the best place to buy.