It's just a normal Class H fuse. Grainger or any decent industrial hardware store would have them. Mine are marked REN-100, and they have replaceable fusable elements inside them which are available in all ampacities, but you need a 100A fuse. (At least, mine is. You should check your bus's Parts Catalog or Service Manual for its exact rating in case it's different than mine.)
I had very bad corrosion where the feed from the start batteries' Cole-Hersee master switch enters the fuse holder, to the point that my engine cut out several times while driving. The DDEC computer was not getting enough voltage to work consistently because the corrosion was causing significant voltage loss under load. The original cable set screw was immovably corroded into its aluminum block, so I bought a similar-looking cable clamp from Home Despot (a Gardner Bender, I think, maybe?) and filed it to fit the fuse holder. Voila, no more voltage loss. You may want to check your fuse holder to see if it, or the cables in and out of it, are corroded - if they are, you'll have plenty of weird electrical gremlins happening, even with a MUI engine. You won't find Class H fuse holders exactly like our buses' any more, and the current ones are a different size and shape, so if yours is bad you'll need to fix it.
John