Need to reload your cartridge box? Powder, Inc. can help you with that small problem. Needless to say, I'm impressed with this company. It offers a wide selection of blackpowder at reasonable prices, and seems to provide reenactor-friendly service.
It is recommended that you use Powder, Inc. for batch orders -- small orders are expensive because of an attached hazardous materials fee in addition to the normal shipping fees. Orders become price effective 25 pounds and over. The automated order section on the site is a great tool to find out what an order would cost ahead of time. They offer a wide spectrum of powders, from Skirimish brand to trusty Goex. Great site -- check it out when your group needs more powder.