David,
Larry L and I bought a bunch of BG707SR plugs on Ebay about a year ago. I think they cost about $3/ea.
including shipping. Keep an eye out there, put it in your search feature. I even see plugs in Canada and you avoid GST. C10S plugs are available in many places, $7.50 or less. They will work OK for about 100+-
hours.
Dan