I have a nice little case that holds my original DS, eight carts, and has a compartment behind the cart straps that can hold a power supply, spare styluses and additional assorted GBA carts that I truck around with me. It's actually this one here:
I bought it for $10 at Best Buy like a year and a half ago and it's perfect for me. I don't carry more than 8 DS carts with me at any given time because I don't like chucking them in the rear but now that I think about it, I may buy some of those 3-cart clear cases and throw the rest of my games in the "handpurse" as well.
Jam it back in, in the dark.