Thanks, Spambait11, for chasing down the link to St. John's Cycles in Bridgwater, Somerset, UK. It seems a shame to order oversees for such a simple item, but absent a domestic source, it may be the best solution. In fact, although I live in Brooklyn, NY USA, it looks like I'll be in the UK at some point late in the approaching summer. Perhaps a side trip to scenic Somerset would be in order ...
In the short term, I'll switch to a 7-speed, wide-range freewheel.
In the medium term, I am going to look for a 36- (or 32-) hole generic replacement rim that has more or less the same characteristics of the Dahon's stock Weinmann TM19 rims. I will then build up a replacement rear wheel based on a 7-speed freehub, and run unmatched front and rear wheels until further notice (no big deal for me ...)
In the long term, I'll swap over to Sun CR-18 rims with a 28-hole rim in the front and a 36-hole rim in the rear. To that end, to repeat, if anyone knows of a domestic bike parts vendor who stocks the CR-18 20 x 1.5 (ETRTO 406) in both 28- and 36-hole drillings, please let me know. Thanks.