just to throw in another wrench in the works: have you consider 650a (ISO 590); Sun offers the CR18 rim in 32, 36 & 40H in that size, and there are tons of tires available. It is still one of the most popular sizes outside of the USA.
26" is generally ISO 559. There is also 650c - ISO 571; almost all really skinny rims and tires now.
Given your current ride is convertible from 650b (ISO 584) to 700c (ISO 622); it should also accept 590 with no issues........