The closest shot size to 1 Buck that's not buck shot is F size in steel shot (probably not save too much money over 1 Buck) or BB in lead shot.
http://www.deltagearinc.com/ShotgunFacts/GaugeChart.jpg
#1 Buck is .30" in diameter and F is something like .22" and BB is .18".
http://www.deltagearinc.com/ShotgunFacts/GaugeChart.jpg
#1 Buck is .30" in diameter and F is something like .22" and BB is .18".