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I think you can buy a single cylinder set. Twenty years ago or so I broke two rings on my first car engine overhaul and I wrote to the manufacturer (Hastings) and they very nicely mailed me a couple replacements, free.
Back in the day, before Federal Mogul bought out Sealed Power, you could get a single set or even individual rings. Don't know about now.
A "W" in front of the part number denotes a set for one hole. At least you have the most common bore size, so even a whole set won't break your piggy bank.
I sold engine parts for years, and the weird thing is that a set for a 300 Ford, which uses the exact same ring set but minus two cylinders, costs more than a 302 set.