How much can flash memory store?

It all depends, you can have a very small flash drive such as a 16 or 32 MB one or a large flash drive such as a 8 or 16 GB. I'm Fairly certain flashdrives can be larger but, I have not seen them in stores.

Go down to your local electronics section at a department store, or a dedicated electronics store, or anywhere they sell computer equipment. They'd have a huge range there.

You can also get flash hard-drives for computers, the biggest I know of is 128 GB.