A quick google and wikipedia search found me
Iron Wind Metals, which (I think) holds most of the old Ral Partha licenses. They actually used to make minis for Shadowrun, and it looks like you can buy boxes of the old minis at their site.
Barring that, I think you're going to have to mod yourself a mini together. A fantasy orc shaman with a machinegun swapped into his hand will probably fit right in. Conversely, if you can find a good
gunslinger model, you could try and mod in
shaman gear. Humans and dwarves are easy to find in most configurations.
A good base Troll is going to be a problem, since the Shadowrun metatype is pretty unique - you might look at the
Warhammer 40k Ogryn models and mod in some horns. Scale relative to your other units is going to be important, though, since these guys are about 1.5* the size of the 40K humans. If your other players are playing with larger models, he's just going to look like a fat dude. If you don't mind him lacking gear, the fantasy line is even bigger and better:
Ogres but doesn't come stock with guns or shaman kit, so you've got your work cut out for you.
If you've never modded a mini before, do a quick google search for 'green stuff mod' and you'll get lots of info and howtos. The short version is, buy a few bucks worth of two-part epoxy putty from whoever sells you your minis, and use it to smooth out the seams when you glue a new head or hand onto a model. For best results, drill a hole into each part and glue a piece of a paperclip into the holes for strength. With some practice, you can re-arm most any miniature.
I hope some of this was helpful.