Most 18th century infantry would not have used more than one or two pounds of lead shot in over the course of a battle. Four shots a minute was considered an good rate of fire for an 18th century musketman, and most infantrymen carried only 20-40 rounds worth of powder and shot, after which point they would attempt to close range and fight hand-to-hand with bayonets.

That said, the real problem for the revolutionary armies was getting enough gunpowder. Lead is cheap, and the smelting of lead has been known since ancient times. In fact, a great deal of the lead and zinc used in Colonial-era North America came from mines (now closed) near what is today Phoenixville, Pennsylvania, about 20 miles outside Philadelphia.

Also, I've been bullets in museums that have "pewtery" color to them, suggesting that the lead shot was quite impure than may have contained substantial amounts of tin, bismuth, zinc, copper and other impurities.