Depending on the exact number of drives you need, you can purchase them either from a retailer for a slight discount or from a wholesaler at a larger discount. A retailer is the best bet for a single person or a very small group, but a larger organization with a greater need for the products can make the best use out of a wholesaler.
It is fairly simple to find retailers, such as Amazon or Staples that sell bulk flash drives on smaller scales. Typically, these sell in small lots of three or large lots of 10. This is typically sufficient for a single user that needs a respectable amount of data storage.
However, if you purchase these flash drives for a large company or otherwise need a very large number of them, it is more economical to purchase them wholesale. This results in paying much less per unit while having to purchase a very large number of units at once. One site that can provide these wholesale prices is PremiumUSB.com. However, you must purchase the flash drives in lots of at least 50. The more that someone purchases in one lot, the cheaper the unit price is. Consequently, the best choice economically, provided that they are all used or resold, is to purchase the largest lot possible for the highest per unit discount.