To secure lead for your PB pistol first choice should be your local metal recycle center. See if they have lead pipe, sheathing, etc. You can usually buy it there a bit over their day spot cost.
Wheel weights are too hard (in my opinion) for BP revolvers, too much stress on loading lever to size the ball into the chamber. I have a few WW balls cast in 1969 or 70 during my learning time, and they are very difficult to get in the chamber. Current pure lead swadges right in.
And no, lead does NOT get hard with age.