These are the O-rings that are missing in almost 100% of the engine gasket sets out there today (circa 2023).
They are $7 each kit, shipped in the USA, which includes $1 that goes back (from me) to these forums.
If you're outside the USA, e-mail for info about the postage difference, which will likely add to this amount a little bit.
This kit ships in a normal business-size letter envelope, so the postage is pretty low.