ORIGINAL: Big_Bird
Andy, I've been flying gas since 1982 and have never seen these gaskets sold separately. You might check with Champion customer service and see if they will sell or give you a small bag of gaskets.
The first time you tighten a spark plug, the gasket flattens. Many times I have pulled a plug to look at the color and have never had a problem getting the gasket to re-seal. Just make sure the seat on the head is clean.
Or you could just switch to plugs with taper seats that don't use a gasket. Except for engines that use the small CM-6 plug, I've converted all my engines to taper seat plugs. A common plug is the NGK BPMR6F, 5/8" hex with std threads. There are Champion equivalents and alternatives. Most seal well in the head with no modifications ... in some cases a slight taper might have to be added to the head for the plug to seal against. I believe I leaned this from Rcign. May not work for you but has worked out great for my engines. These plugs are also slightly smaller than their gasketed partners. May or may not work with all plug lead metal caps.