I've purchased bulk usb flash drives from www.usbsuperstore.com before and they were quite reasonable. In fact my group bought 400 about a month ago. I couldn't find a lower price out there. good luck!
We are a UK based supplier of promotional flash drives which can all be printed with your logo. We have a huge range of 100+ models available, which come in a range of memory sizes from as small as 128MB upto a huge 32GB!