Yeah, that seems to be the rub, spend $88 per engine for three O-Rings worth about $1 each but know they fit, or do the labor of finding aftermarket O-rings.
I don't want to start thread creep in my own thread, but marine engine companies really bug me with stuff like this. I don't mind Yanmar making some money on parts but when they give you a royal screw job like this it really makes you want to avoid buying from them. How about pricing the rings at $10 a piece? They still acheive a ridiculous profit margin but I'll pay it. When they take it too far I just don't buy that part from them at all, so they get nothing.
I just replaced my air filters, a little foam ring about 2 inches wide and 12 inches in diameter, $33. Geez.
It's not the money so much, I can afford the $88, it's the principal.