You could probably find something cheap that would work at a thrift store. I've bought so many official Nintendo carrying cases at thrifts (all different ones, no less; I don't buy the same one twice) that I'm starting to turn them down, and there's always tons of other various foamy cases that were intended for all sorts of different stuff. I rarely see them priced any higher than $2.99.
I picked up a carrying case for a Mini-disc player back when the GBA LT released that was a perfect fit. It holds the handheld, AC adapter and at least a half-dozen carts quite nicely. And it doesn't scream GBA SP AND A BUNCH OF NICE SHIT all over it.