I'm surprised you describe the 51R as rare. Here in the Portland/Vancouver area, I've found it at every local store that carries car batteries. It's in millions of Hondas, and probably other makes as well.
OTOH, if you claim it is small and overpriced, I'll definitely agree with that!
2013 Pearl White SV, bought used in 2017 with only 5100 miles. Vancouver, WA.