For everyone looking to make leather type parts or leather looking things:
I've done alot of work with leather; I've built armor pieces, customized jackets, you name it. And for things like this where you need to kind of dial in the right sizes for a custom build, I've always used cheap craft store vinyl, both as a sacrificial test-run type material and for templates where it doesn't matter if it gets trashed.
Usually I will make a quick-fast built of whatever it is with the vinyl just to make sure everything works, and then do cutting on the real leather (because there's no room for error there) afterwards. I've found that cheapie vinyl works great for costume parts, and I would suggest it as an alternative to digging through racks of clothes in thrift stores and looking for real leather parts. It usually runs around $3 a yard, so it will end up being way cheaper than even a used jacket.
Vinyl is inherently water resistant, and you can put a sewing needle through it like butter; whereas trying to sew leather will kill your hands pretty quick depending on the gauge of it. And for Tron armor, a few coats of Plasti-Dip will have it looking rubberized in no time (which is how I plan to build my Rinzler armor).
Note: I am talking about vinyl, not PVC. Think more like cheap computer chair upholstery material and less like shiny sexy clubwear material.
As for everyone wanting to go the distance and use real leather from scratch:
There are alot of places online where you can buy wholesale leather pieces; there's even some places where you can literally buy the whole entire side of a cow. If anyone is that ambitious, I would suggest Tandy Leather Factory
Tandy Leather Factory - Leathercraft and Leather Craft Supplies, as I have gotten huge pieces of leather for cheap, as well as high quality hardware (rivets, buckles, spikes, etc) from them. If you see any real leather heavy metal or goth type wristbands or armor in your local shops, chances are the parts came from these guys.
And for people wondering about leather thickness and gauges, leather is usually measured in ounces. for Tron stuff I would go thin; very thin. Unless you have pro leather working tools, you won't be able to work with armor grade leathers (which run about 1/8" thick, and is 8-10 oz). You will want a "garment leather" grade that is 1 to 3 oz. That should be light enough to work with, but sturdy enough to make convincing armor that's not too floppy or swishy.
There's also ebay sellers that sell leather parts; usually from upholsterers who have leftover pieces from large sets of furniture, sellers from Harley shops who have leftover pieces from making jackets, chaps, and saddle bags, and an interesting group of sellers I've found are equestrian shops who have leather pieces left over from making horse saddles and other horse related things.
Judging by how this thread has quickly become the de facto place for Tron costume stuff, I hope this helps everyone somewhat in trying to get leather or leather looking pieces!