Buying Cheap shot shells to recover the lead from them instead of buying lead in bags in either 10# or 25# quantities is a very expensive " solution " to your problem. At. 1.99 per box of 25 shells, ( on sale, and usually in 1 oz loads, only) will give you a cost per ounce of shot at 8 cents. 25 lbs. of shot at 8 cents a pound is $32.00. Certainly you can't practice much at those cheap shell prices, which you would be able to do with the purchase of a large bag of shot.
If you do practie, by going to skeet or trap ranges, or even to sporting clay ranges, you will find other shooters who reload, and they would be a much better and cheaper source of small quantities of shot, since they tend to buy in quantity, and find where the best prices are anyway. Most hunters who reload had a bag or two of #6 lying around for their own use, as its a common shot size. I don't, but I have #5, #7 1/2, and #8 shot on hand. For rabbits, the 7 1/2 shot would do the job and then some. Considering the brush that rabbits like to inhabit, a 20 yd shot is a long shot, and you don't need a heavy pellet like #4 shot to get through it to kill the bunny. You are not likely to be able to see that rabbit at a distance where the #4 shot give you much of an advantage. Only when you have a good layer of snow on the ground, to mat down the grasses, and weeds can you expect to see a rabbit out there, and then the heavier shot can reach out and bag a bunny that you might otherwise not get.