You're in luck, because flash drives are commonplace and dirtcheap nowadays.
The only real thing to watch out for is getting a flash drive with USB 3.0 (instead of USB 2.0).
Be careful when purchasing a Flash drive that doesn't specifically state USB 3.0, because if it doesn't, it's most likely [USB 2.0].
The difference between 3.0 & 2.0 is substantial, BUT you must make sure you can actually benefit from those speeds (do you have a PC with USB 3.0 ports? check your laptop/PC specs or look for USB ports with a 'red coloured' interior..is the easiest way to spot a USB 3.0 port).
32-64Gb is usually what most people go for but it purely depends on your requirements - what will you be using it for?
You have the choice of:
32Gb
64Gb
128Gb
256Gb
I am sure overclockers sell these, but I am not in a position to browse and recommend one at this point in time (someone else might be able to though!).
If not, I highly recommend purchasing one from an established company and not from 3rd-party sellers of auction website (too many usb drive scams going out there at the moment...it's hit & miss).