These are basic single-speed freewheels made for BMXers, but no less suitable for our ilk. Your local shop probably doesn't have these sizes. Certainly not the 20 and 22, and that's why we stock them. The 18 you can get most anywhere.  They all work with either 1/8" or 3/32" chains.

We'll get whatever brands we can. If you order a 20 or 22, it'll likely be ACS (a biggish name in BMX), and if you get an 18, it'll be either Shimano or ACS. For a budget, basic ss freewheel, it doesn't make a big diff. If you want to get fancy, pay 4x as much and get the White Industries 2-speed versions.

Here's just the thing you need to take that old one off.