Most muffler shops can do it, since they are very familiar with welding. Just go thru the yellow pages. If they do not know what SFC's are, say "thank you" and hang up ... most will know what you are talking about. Just make sure that they weld them in "loaded" (with the car on a DRIVE-ON lift). Price should be about $80 for regular length.
Full length go all the way to the firewall, while regular length are a somewhat shorter. I think most people here have gone with regular length - the big hitters are from Steeda and Kenny Brown. IMHO, regular length is fine for most applications. But if you will be seeing the track a lot, then you are likely better off with full length. Typically, full length can be a bit more expensive to buy and weld in, and can also invoke some under-car clearance issues if the car has been significantly lowered. The big vender for full-length SFC's is Maximum Motorsports ... they have many happy customers. Regardless of what you go with, just stay away from venders you have never heard of.