'bent shops, like hostelshoppe.com ($$$), tend to have some things that will work on 20" folders, since some 'bents use 20" front wheels. I've found bikepartsusa.com to have lots of 20" stuff. e.g.,
http://www.bikepartsusa.com/view.asp...e&f_c2=20+inch
The 20" stuff at NB and PB is mostly BMX related, but occasionally parts that work well on folders are on offer, e.g., Pyramid 20"x1.3" tubes on NB.