You can get them dead cheap on amazon, since they’re basically just a couple of free-spinning hardened steel star-wheels in a handle. Just make sure you get a dressing wheel and not a dressing tool - the dressing tools are basically just a bit of box-tube with diamond dust glued on, and don’t work nearly as well, as well as being harder to use. A dressing wheel, if you do managed to wear through your wheels, you can replace them cheap as chips, since they’re basically just some hardened steel sheet. Mine set me back about fifteen bucks, it’s violently orange, I use it maybe twice a year at most, but it makes my grinder so much more pleasant to use.
Just make sure you wear some goggles and have the vacuum or dustpan ready, because as I’m sure you can imagine, it produces a LOT of stone dust, since it’s basically just pulverizing a thin top layer of stone as well as knocking loose all the loaded-on metal.