As a former radiator shop tech (40 yrs ago)....

Visit pretty much any non-chain radiator shop and they will most likely give you the correct size plugs for free (or very cheap) if not they will probably be able pull the tank off the header plate and install a manual tank. Radiator tanks are sold by manufacturers to shops only, so you won't find them outside that chain.

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course