I spoke to CH Ignitions and placed an order for Rimfire 1/4 x 32. The airmail charges are the same for 1 or more plugs, so I went for 4 plugs. Based on my flying it should last me a while. But these plugs are fragile, the manufacturer (Mr. Keith Baker, BME) cautioned me a few times. I'll check with him on the mods to 10mm.
Thanks for your help and suggestions.