So are you looking for the carburettor float bowl O ring ?
Welcome to the wonderful world of independent mower repair technicians.
First choice would be to find a workshop only repairer, take the carb to them & see if they have a work around.
I do this sort of stuff every day of the week for customers because "they said it is not available & I have to buy a new mower"
Other than that an engineering supply store, the type that sells bearings, belts pulleys, gasket material & O Rings.
Usually they will be able to find a replacement but most times they will come in a pack from 10 to 50.
SO buy the pack , keep a couple in a sealed glass jar then sell the rest .
As it is NLA everywhere you should be able to get rid of the rest at a nice little profit to cover your time & effort.
I have been doing this over the years and it is becoming a very profitable side line.
People will happily pay $ 5 + p&p for a 50¢ O ring if that is the difference between getting another 3 years out of their old mower or buying a new one.
At the very worst you can cut a ring out of petrol proof buytl rubber sheeting if you can get some in the same ( or slightly larger ) thickness.