It's not too hard to build a case and most of us are forced to do so occasionally to accommodate some odd shaped instrument we've built. There are catalogs and online stores that sell all the hardware you need. When you figure in the wood and time involved, most people would rather put the money toward the instrument than the case.
If it's a one-off sort of thing you're best bet is in cruising your local thrift shop for suitcases and equipment cases. You'd be amazed at the variety they have and the customizing possibilities are endless. They can be easily lined and padded to suit your needs. I've bought vintages suitcases for a few bucks just for the hardware.