We usually make our own. Just buy a piece of plexiglass, and cut it round on one side to fit around the base of your router. All you have to do is figure out the radius of the circle you want to cut, measure from the router bit (depending on if you are cutting the inside or outside of speaker rings, allow for the bit's thickness) out to a point on the extended piece of plexi. Now take a small drill bit and drill through the plexi at that point, and into the center of the circle you want to cut out. Once the drill bit is through the plexi, and in the wood, take the drill bit out of the drill, leaving it in place. It will act as your pivot point to cut your hole.
Make sure you drill holes in the plexi in well spaced spots. Otherwise it will become easy to break. After a while you will have to many holes, so you will have to make another one. But since it only costs a couple bucks, it's worth it. Plus it's kind of nice to be able to see through it. The solid ones you buy must be frustrating sometimes to find your center mark on the wood.
Easiest way to make and use a circle jig is by using a router guide collar attached to your router.
This is a 5/8" guide and a 1/2" bit installed on a Makita router. There are other sizes available.
To "build" the jig, simply cut a scrap piece of material ~6" wide by ~16" long. Make a reference mark 3" from the end of the jig at the center across its width. Take a compass or trammel points (or anything to mark a larger radius), draw a radius reference arc using the first mark as the center. I usually use 10" to make it easy to measure radiuses for drivers later on. The arc gives you the ability of using the whole jig to mark and drill centering holes.
Using a nail or punch, make a starting hole for a drill bit. Tap a hole at the first reference mark. Using a 5/8" (in my case, the size of the guide collar) forstner bit, drill a thru hole at this mark. The jig is complete.
To use the jig, simply mark the radius you want (remember that the reference marks are to the center of the bit). To make an 8" hole, for example, holding a ruler on the jig, position the ruler's 10 1/4" mark on the reference arc with 0" towards the 5/8" hole. This gives you outside of the bit. Make a mark at 4". Drill an 1/8" hole thru the mark and thru your baffle and use the loose drill bit as a pin. Flip the jig over (in case you don't drill the center hole at a perfect 90 degrees). Now simply drop the router on the jig so that the collar drops into the 5/8" thru hole. You're ready to route.
The beauty of this jig is:
1) It's DIY!
2) It's cheap.
3) You don't have to attach it to your router every time you want to use it.
4) You can keep a firm grip on the router because the collar rotates on the jig. You don't have to walk around the workpiece.