Here is the Flash Drive that I used. You can get a pretty good deal on it through Amazon, but since it is Amazon, they like to change prices on people, put specific ads on their pages, and whatever, so I can't give you a direct link. But if you just go to Amazon.com and search for "Viking USB" it will come up with all the different memory sizes. Mine is 128mb because I did not want to spend money on something that may get damaged.
Remember, this is the only Flash Drive that I am sure will fit (with proper adjustments) in the Lego brick. Some other flash drives look smaller on the ouside, but their components(chips) are bigger, so they may not fit. Also if you want to do any of the other mods involving USB Flash Drives that I put up, I use the same component for every one.