Raspberry Pi's generally don't have volume discounts, because the companies (2) that have the manufacturing rights sell them pretty much at cost already. Your best bet for savings is buying bundles with accessories, companies have a little more wiggle room for discounts then. (I found this out from an employee of Element14).
The cheapest place to get them is usually Element14, their parent company is Farnell which is one of the manufacturers.
Gadgetcat has a bunch on site and typically what we do is mark up items 10% from the wholesale rate. For the Pi, that brings it to about the same cost as purchasing from Adafruit or Sparkfun, with the convenience of local pickup/delivery. They can be picked up at the lab or delivered in the Orlando area, or possibly Brevard Co.