Kamloops, thanks for the great info. Did this today, and it needed it
When I put the toyota part number 90301-22013 in google, the first result was "68288"
Looked on line, couldn't find a better price after shipping than what the dealer had, so I ran up to the toyota dealer and bought the O-ring for $4.98 after tax.
Didn't have any trouble getting the tensioner out, it just slid right out. The old O-ring was thinner than the new one, on the tensioner, it didn't stick out beyond the diameter of the tensioner. The new one did and was a lot snugger going back into the engine.
The only trouble I had was that the pin that holds the ratcheitng part in place was offset, it stuck out of the side of the tensioner far enough that I couldn't move the hook into position to hook the pin on the plunger, so I had to set it against a solid object and drive it back in.