www.ordertree.com is another source. It will cost you though. $6.10 for the ring and $6.95 shipping & handling. Ouch!!
Anyhow, they stock thousands of parts. I ordered hard to find tablesaw drive belts a while back from them.
Also
www.sears.com has all the parts for the Ryobi trimmers. They want $5.49 for the ring and $4.99 shipping & handling.