Most any brand of USB disk should do fine. Just watch out for these factors:
1) the disk must have enough capacity to hold everything that's on the B2. Check the Status window in the WebUI to see how much space is used.
2) the disk must be legitimate. There are an abundance of fake USB sticks out there, mostly on eBay. Buy from a reputable dealer.
3) the disk must be formatted as FAT32. No other format will work. The B2 can format the disc for you, and that is the preferred way to do it.
If you need further elaboration on any of those points, just ask.