I have no experience with Tern, but I do have experience with Dahons, still have a couple. Used to be a member on the old dahonforums.
I don't know how Dahon is doing these days with parts, but back in the dahonforum days, stories of parts non-availability were legion. Apparently it had something to do with the numerous model changes every year and a meager store of parts to supply all those different models. A lot of those parts were proprietary, so you couldn't source them elsewhere.
Anyone know how Dahon is doing parts-wise these days?