Here is a post of interest with a whole lot of various flash carts:
http://imgur.com/a/gzkD8
This group represents about a decade of programmable multicarts. Some of these you can google and find for sale.
This is also an interesting and very long running (and still active) thread from AtariAge.
http://atariage.com/forums/topic/127752-flashcart-and-multicart-list-all-systems/