I used a Radial arm saw, but a chop saw, miter saw, or table saw should work just as well. Depending on the size and thickness, buying the circles can get pretty expensive. Here is how to make them out of any piece of wood that you have.

Step 1: Material, BOM

you need a couple of wood screws, a nail with the point cut off, and a piece of scrap wood. Also about 3 minutes.

Drill a hole in the scrap wood about in the middle and push the nail up through the hole. I cut off the head of the nail so I don't poke myself as I work with this jig.

Now just place the jig such that the nail is at the distance to the blade such that at 90° the length is the radius of the circle you want. screw the jig to the tabletop with the wood screws.

Then find the sorta center of your stock that you want cut your circle out of. and drill a hole there that you will fit over the nail.

Step 2: Just Keep Cutting the Corners.

At this point you are just going to cut the corner off, then spin the stock, and keep repeating. When you get it down really close to round, you can actually hold the blade out and spin the stock to make it almost perfect.