Where can I buy odd lots of walking shoes?

If your feet have more than a 1½ size difference, then getting mismatched shoes would be better for your feet’s health and protection. However, if money is a problem, you could try just fitting the shoes to the larger foot. You can buy insoles to take up room in the smaller foot’s shoe.

Shoes For Different Sized Feet | Healthy Feet Store
www.healthyfeetstore.com › different-sized-feet

I had a friend 50 years ago with his left foot more than a full size smaller due to club foot and multiple operations. There was a group that he was in that had shoe swaps like you’re talking about. By the way my left foot is a half size smaller than my right. I just buy shoes that fits my right foot !