The original PlayGear Pocket has been out for a while now. Long enough that those with the newer slimmer PSP have found ways to pad the original case so that it works with the new PSP.
Finally they have a version for the newer PSP as well as the DS. The case for the DS will hold space for up to four DS cartridges.
They haven't been released just yet, but you will likely see them pop up soon. Once they do you can expect to see the case for the PSP sold for $15 and the PlayGear Pocket DS for $20.