It appears to be a Bianchi pistol belt. I think it was called the Texas Ranger, or something like that. Since it appears to have "keepers", it may be a police duty belt.
The holster in the pic is an Eagle SAS MK something another... it's one of the thigh mounted versions that has been raised up closer to the belt. Notice the pistol mag pouch on the front of the holster. These holsters have been in production for the past 20 years and there are several copy-cat versions made by Blackhawk.
Looks like a Bianchi to me too. It's a Texas Ranger style belt. Kits for this type can be found at just about ANY leather shop, and the two snap keepers are a cinch to make.
If anyone's interested, I may be able to punch a few of these out...
I've found the correct version of Eagle SAS MK holster for Jayne's gun. The one EchoLeader posted is pretty much the same, it's the SAS MKV but I am pretty sure the one on the show is the SAS MKIII which would require no modification since it only has one leg strap.
Or possibly the SAS MKIII Airborne version which would just require removing the extra strap for holding the pistol in.