Try an acoustic guitar gig bag or a gig bag for an ABG.
If it doesn't have to be a bag, this case is not bad for the money (I bought 3). http://www.rondomusic.com/product4952.html
Note: the photo with the case open and a jazz style bass is deceiving. That case is for the SPB-1100 which is a 32 in. scale, EB-2 shape bass.