I'd recommend buying one tube of "bore butter" or similar lube packaged by Thompson Center or Traditions or CVA so you can carry a good amount of lube to the range conveniently. After you use up the lube, you can always refill it with crisco that has just barely melted. Of course you can always do this with a old tube of toothpaste, or buy a nice brass grease holder from Dixie.
Also, I've never had good luck with cappers when using them on factory nipples. It's best just to put them on by hand- so you can pinch them a bit to keep them on the nipples a bit more snugly.
I do like flasks simply because you can carry only a fraction of the amount of a full can, and in that way they are safer and much more handily carried.
If you want a superb nipple wrench, the best one is from the Possible Shop:
http://possibleshop.com/s-s-nipple-wrench.html
It won't strip like the soft metal in most "period" wrenches and will help loosen the most stubborn nipples.
Real black powder is a good choice to start with, but any of the substitutes will work well too.