Hey there CTF,
As someone who has researched and built this costume (over and over until near perfection) for nearly 30 years, I can tell you, that since this is a one-time use, just keep it simple.
Get yourself a pair of black jeans, and pin-striped button-up shirt (don't worry about the direction of the stripes), a simple pair of (tan preferably) cowboy boots, a black bandanna, any cowboy hat with a relatively flat crown and brim (hopefully brown), and a few Backwoods Cigars. You should be able to find most of this at Walmart or any thrift store.
That only leaves the Poncho and Sheepskin Vest.
My first Poncho was made of forrest green wool of a medium weight. The pattern was created using white felt and Stitch Witchery. It worked well and I still have it today.
For the Vest, I used faux sheepskin (really inexpensive) and sewed it by hand.
It made a really good start and fooled everyone who saw it.
The rig I used ran me $12.50 and I used a Colt Army (wrong gun) that I built from a t for $75.00.
Feel free to email me if you need advice. Maybe we can talk on the phone.