It's wierd, I ordered this
Plasma Cutting Guides - plasma cutting guide w/1.007 bushing - Power Plasma Cutters - Amazon.com
from amazon, and it came with a longer rod version of the one Shield posted thrown in. I'd planned on using the "chariot" and making my own circle burner, no biggie, but "free is a very good price", as they say.
The combo is pretty versatile, all the hardware is 1/4-20 so you can use one or two wheels, use a wheel on the opposite side of the rod/center pin, make your own shorter rod for tighter circles - The rod that came with mine is just over 18" long, so you can do up to 36" circles if needed.
My ZTR has kind of a wimpy deck on it, so to tie the bottom front/rear corners of the discharge chute together I wanted to use a piece of 4" wide 1/4" flat bar - but I didn't want any collisions with the right side blade, so I turned the deck upside down, bolted the piece of flat bar on, stuck a socket on the spindle nut and a wierd adapter I had from who-knows-where, and used that as a center for the burner - set the radius to about 3/4" bigger than the blade radius, cut it out, ground the edge so grass wouldn't hang up as much, and it's been solid ever since.
Somewhere I saw a guide similar to mine that a guy had made, he used a couple of cutter wheels for a tubing cutter. Might be OK if you had access to used ones, but I priced new ones and you wouldn't save anything (didn't check HF for crap tubing cutters, that might be something to check on... Steve
Here ya go, this one even comes with two extra wheels
Pipe Cutter - Get this Pipe Cutter for less at Harbor Freight