Well, you really kind of do need a pair to practice. Reason being is rubber, which is found on pretty much all normal shoe soles, is meant for one thing: Grip. Grip is the enemy of grinding though. You can try the wax, or if you are in winter try going across an iced rail or something, but if any of that rubber hits the material that isn't wax or ice, you have a good chance of being in a world of pain.

At the moment, you can get a pair for low cost, cuz there's a ton of them on ebay. 40 bucks isn't expensive for ANY pair of shoe. And plus, these aren't like heely's, meaning that they feel like normal shoes (heely's have bigger bottom sometimes uncomfertably firm bottoms). I would get a pair of soaps even if i stopped grinding, cuz they fit my feet so well and feel great, not to mention look nice.