Dahon Boardwalk 6 conversion
You don't need a Dahon specific wheel to upgrade from 6 speeds to 8. Bicycle wheels (like practically all bicycle components) are very standardized.

What you need is any wheel with:


The correct rim diameter
Acceptable rim width for the tires you want to use
Correct hub width/dropout spacing
8/9/10 speed freehub (slightly wider than those used for 7 speeds and below)


A difficulty I can foresee is that Dahon 6 speed bikes might use an older style 120 or 126mm wide hub rather than the now standard 130 or 135mm. An 8/9/10 speed hub that takes a cassette  will not be available in that width. Dropout spacing is very easy to measure with a ruler so you should check what you bike has.