maybe check Craigslist ?
or kids thrift shops ?
I'm in the same boat as you, with 3 kids, renting shoes each time we bowl for free, really adds up.
Our place has a $25 card per person for shoe rental for the summer, but still, I'm going to check out some other places first.
Actually, I'm going to call and see if shoes are even required, otherwise it may just be socks with grippers.