I used to work at Sportmart, so i just took em from other shoe boxes, lol.
Maybe try asking a shoe place if they have any to get rid of. They were all over the floor at my work cuz of sloppy ass customers. We would throw tons of it away each night.
If that doesn't work, $13 for 150 doesn't seem like too bad of a price. U really need just maybe 8-10 max per shoebox. Not a bad investment, cuz that silica gel wicks away moisture from the shoes.