Where do you get the R4?
lots of places online, just have to watch for ones that sell legit ones.
The R4 is pretty outdated though, and there is a high chance of getting one that will brick your DS, so if you're looking for a cheap flashcart (but still works perfectly with any commercial game or homebrew application/game), try something like the AceKard 2, Edge, or something with more features like the CycloDS (with like slowmo and real-time save state options). Do some research and look up whatever looks like it fits with your needs.