I called Ryobi customer service and they sent me a new belt for free.
Mine was about a year old, but they didn't care, and didn't even ask
for any proof of purchase. It took a couple of weeks, but free is
good.

Be sure and keep the belt tight enough that it doesn't skip cogs when
you load it down. On my unit, that means tightening the belt until the
unit stops somewhat faster when you turn it off. It seems like that
would be too tight, but it works fine this way and I can stall the
motor without jumping cogs. It's the jumping that eats up the belt.