Etsy user AyameMizrahi has made some nice felt cases for your non-DSi XL DS needs, all of which are available at her page here. Some of the cases she’s made include the Katamari-inspired Miso and Prince, Ninjatown, and Chocobo. Each are $15 and will fit your 3DS, DSi, and DS Lite. They won’t fit DSi XLs.
I really like her designs; if I had the money, that Miso case would be all over my DS Lite. And if you have a favorite cousin from the Katamari games that you’d like a case of, you can message her on Etsy and see about getting a custom design made.