For Metal Studs:
Some mounts come with several types of bolts for various purposes. Also you may call the mount manufacturer that may have bolts. IF not, I'm sure you can find some that are a similar size at Home Depot, Lowes or other hardware store. Drilling is the same, you just need the proper bit.
The difficult part isn't the mount but the wiring (as in where do you put the wires?). Even so, there is usually a lot of room between the face of the fireplace and the actual exhaust tube. Properly done it is still possible to do a concealed wall mount.
Special Jacks and such. Radio Shack actually sells decent stuff. Why don't you try their products first? Fry's Electronics carries a lot of stuff and I would suggest you try the forum sponsors above. I have had success finding most of what I need at Ram Electronics.
Studs off center:
Many mounts have horizontal adjustment. This means that the mounting plate does not have to be centered on the studs. The mounting bolts still go into the studs, but you can slide the mounting plate left or right a few inches to get teh mount centered on the wall. I think I have an omnimount which had this feature. You'll also want to look for a cutout in the mounting plate for your outlet and other wiring.
Contrary to the Philips commercials, you cannot hang a plasma on the ceiling (so that you would be looking at the ceiling to watch tv). Your set manual will tell you not to do this. I have no idea why, I recall seeing the reason in another thread some time ago, but I knwo you aren't supposed to do it.