I highly recommend learning a CAD program to make your own. Boxes are generally pretty simple, and you can really go to town to make custom boxes as you learn new CAD techniques.
Another option is to download real models from manufacturers like Hammond. A 3d printed copy will never be as good as the original, but it can serve as a prototype while developing. If I want something tough, I'll design around a metal enclosure, but during prototype stages, I'll use 3d printed version to get all the holes (mounting holes, switch and connector cut-outs, etc) positioned correctly - I usually cnc mill the enclosure once I'm ready, but printing out a 2d plan and sticking it to the real enclosure as a drill/cutting template works too.