Where can I bulk-buy cheap parmesan?
The price of genuine-sealed-and-certified parmagiano reggiano is quite high and fairly consistent, especially for more aged varieties.  I think your best bet to save money will be to use a similar parmagiano reggiano cousin, such as grana padano, romano, or a quality Argentinian reggianito.  As a second-tier approach, you might look at domestic Parmesan, but use caution as it may not have the same quality as a one of the Italian imports.  

As for genuine Parmesan: you're buying at least a couple hundred dollars of cheese, and this gives you some bargaining power.  Buy or ask to taste a small sample of whatever you're considering, and try to negotiate; managers may be willing to offer you a bulk discount.  I'd gather a couple quotes from cheese shops, try prices at your local Costco/Sam's Club, and talk to the upscale grocery stores in your area.   

As far as pricing goes, I've seen genuine-seal-and-everything Parmagiano Reggiano on sale at about $12-13/lbs at my local Southern Season.  I generally expect to pay $18-22/lbs at upscale grocery stores for Parmesan, with remarkably little variation between stores.  Your standard grocery stores tend to sell small blocks of inferior-quality and freshness Parmesan for similar prices. 

In contrast, grana padano runs around $12-13/lbs on average at my local Trader Joe's, and might be available for under $10/lbs with a good sale.  I usually buy blocks of that for general cooking use, as it is close enough in taste and much cheaper.