the body is fine, worry about the fuse.
Can anyone tell me what current hand grenade casings/mechanisms are made from?
I need some that are non-magnetic/non-ferrous
Above the discussion is only about the graneade body. the mechanism in many grenades consists of a springloaded hammer, held in place by a spoon (the outside handle going down the side of the grenade.) which is in turn held in place by a pin. This mechanism, combined with a fuze and detonator is called the Fuze assembly. the rest of the grenade is the filler inside the body/casing
Exploded (ahah, ahah, view):
http://maic.jmu.edu/ordata/FullImage.asp?Image=images\E\E3981U02.JPG
On some designs there are some sort of extra safety, but mostly you pull the pin with your weak hand, holding a good grip on the grenade with the strong hand. When you throw it, the hammer spring pushes the hammer and the hammer pushes the spoon up and over and then it falls off. the hammer continues and hits a fuze, igniting it. After several seconds, depending on model, the fuze will detonate a primer which will detonate the main charge.
The problem with non metal grenades is that the body can be made out of plastic and ceramic balls, but the fuze assembly is mostly metal. If you want that part to be nonmetal, I would suggest a jury-rigged design utilising a friction fuse. Much the same as in the venerable potato masher, the german stick grenades:
http://cdn.instructables.com/F8C/YE79/FUVSGTQF/F8CYE79FUVSGTQF.SMALL.jpg
http://upload.wikimedia.org/wikipedia/commons/d/d9/Steilhandgranate_Schnittmodell_db.jpg
This eliminates the need for metallic fuse assembly, and if they use a paper/plastic-covered primer they should be home free.
Two problems though.
1. If they juryrig a fuse it might give their position away as it would smoke and spark in the air.
2. There is a reason why soldiers generally do not muck about with this stuff. It gets unreliable fast. It is very difficult to get a predictable time on a manually made fuse. It is also very difficult to get a predictable bang out of a home made primer. Generally: avoid things that go bang when it wants as opposed to when you want.
Opinion, not fact: If at all possible, your protagonists will probably want to smuggle the fuse assemblys and primers and count on them being small enough not to be noticed. the grenade bodies are main thing to make invisible.