So I eBayed myself a Husqvarna 310 and it came with none of the accessories
that are indicated in the normal packing list for the model. While
wrangling with the seller, I started searching around, locally and on the
web, and haven't been able to find anything at all about purchasing the
accessories separately.

A Viking dealer can get any of the accessories you need or want; some will
be worth getting genuine Viking parts for (e.g., machine feet from the
manufacturer usually work better than generics), some
will be cheaper from other venues (humpjumper, button reed, etc.)

Might try a wish list listing at http://www.rummageads.com