Ryan, I'm not sure where you get 10" rims, but my Mom's Honda N600 takes 10" tires and used to have trailer tires on it,
we replaced them with these and it's a vast improvement, for my Commuti-car I have 145/70 R13 tires that I got from a Zenn NEV dealer, to get them I talked to
Tim@greenautos.com, they might have 12" tires as well seeing as how no normal tire shop carries 145/70 R13.
Next set of tires I need for my Citi-car I'm going to get from Chris Schneider at Honda Motorwerks in La Crosse WI, they sell Columbia NEV's with continental ecotrac 145/70 R13 tires and he claimed that his parts manager can at least get that size, so it might be worth checking with them as well.