I would be looking for a reputable local IT recycler that can supply good pulls at capacity. I would also advise sticking with the same manufacturer and model ranges. Otherwise you will be collecting disks piecemeal for the next couple of years
"cheaply" may be problematic. Buying that many HD's is bound to get you more bad ones than you'd care to deal with unless you can insure they are pretested. Most seller use stupid terms like "great shape", "excellent", "tested", etc. which mean absolutely nothing.