The only other 10mm resistor spark plug I know of is made by Denso, but is about 1/2 inch longer..The Champion is a relatively new addition...If you have another 10mm resistor spark plug I would like to know the brand...The Champions are available from BH Hanson in Las Vegas.. Horizon Hobbies has them for more $$..They like to add to the retail price because they think people can't get things elsewhere...Horizon price is $10.50..
sparkplugs.com has them for about $5.00...
Any other 10mm spark plug with an "R" in the number will be OK, but might not have the thread length of the RZ7C and the Denso...
The NGK CM6 will work, but is not resistor and has shorter threads..The red spark plug boot is made for the RZ7C and won't fit the CM6 very well...