Any farm supply store, tractor supply, coastal, or you can order online. Just for kicks I will tell you. I also go the other way and use absorbine jr. made for humans on my horses. It is an easy to apply liniment for sore legs. You could try that product too.
I do mix horse lineament in a lotion base and rub on my feet before bed every night. The cooling effect helps me get to sleep most nights. I do occasionally still end up in my chair with my feet on an ice pack, but it is fewer nights than ever since I started doing this. I look for one with peppermint oil in it when shopping for the liniment, I will use different ones and I find those are best for me.

Horse liniment
is at Western equipment stores, saddle shops, or a veterinarian.....also I don't see any sulfa in mine as someone mentioned.... Madge