Bass Pro is oriented more towards the inline crowd than the traditional shooter, so supplies for patched round ball (prb) shooters are sometimes lacking there.
Powder: two choices for real black powder:
Graf & Sons and
Powder, Inc.
There are others that require the order of at least 25 pounds, such as Track of the Wolf, but Graf's and Powder Inc. are the most flexible in my opinion. The choice of which to use is an economic one depending on how much you want to buy.
Shooting patches: almost any online bp product retailer can provide reasonably priced lubed and dry shooting patches. To name a few, in no particular order:
Track of the Wolf,
Thunder Ridge,
October Country,
The Possible Shop,
Dixie Gun Works,
Jedediah Starr,
Cabela's,
Log Cabin Shop,
Midway, USA,
Midsouth Shooter's Supply...I'm sure I've missed a few. As far as thickness is concerned, you need to develop that on your own by shooting some groups with different thicknesses to see what your rifle likes. With a Lyman GPR in .54 cal I'd start out with a .530 ball and a .015 lubed patch, then try .018 or .020. Buying pillow ticking at Wal-Mart (just ask the lady and be sure to specify 100% cotton, buy no more than 1 yard) will work just fine for the thicker stuff if you can't find it on one of the links. Taking a set of mikes into Wal-Mart and miking the material is a hoot...
You can also vary the overall projectile/patch fit in the bore by going to 0.535 balls with different thickness patches as well. Lots of things to try - lots of opportunities to make smoke.
There are lots and lots of threads on cleaning/swabbing between shots. Almost as many opinions as blowing down the barrel or carrying on empty (inside jokes - you'll get it someday). Paul V., Zonie and others have provided some excellent advice - mine is no better, but I use a mineral based oil lubricant like Ballistol, Butch's Bore Shine or T/C No. 13 on a patch, run down to almost the bottom, turn the patch over and run down again to the bottom, then follow with one dry patch. People use Windex with vinegar (the store-bought mixture), water, spit, moose milk, etc. etc. etc. The list goes on. Almost all work just fine.
Enjoy!