Most sporting goods stores that sell reloading components will also carry shot. I buy mine wholesale from the manufacturer, but that may not be available to you. I'm buying 1,000 pounds next week and the cost at the plant is $64.00 per hundred pounds. I've seen it in stores lately for as high as $24.99 per 25 pound bag, and for as little as $21.99 a bag. I'm getting it for $16.00 a bag, but that's because I'm buying half a ton at a time.
You also might check if there are any trap or skeet clubs in your area. They sometimes buy in bulk and sell to their shooters at a reduced rate. Our club mines the shot and trades it back to the manufacturer. For every ton of mined shot we take him, we get half a ton of new shot in return, minus 3% for impurities, though we clean it pretty good before we take it to him.
Hope this helps.
Fred