Does anybody know of a source from where old and/or outdated, behind
the ear hearing aids can be purchased?

Hi. I know someone who used to have a pair of those (Siemens brand)
and eventually switched to the smaller, less conspicuous, in-the-ear
ones.  But even the behind-the-ear types had to have a custom fit for
the earpiece. Some cheap behind-the-ear models today are 'one size
fits all' and I think they basically amplify all sounds (I believe
that the more technologically advanced hearing aids today allow you to
amplify only certain frequencies so that you get the right sound
spectrum, not just an amplification of all of it).  If you go to
www.ebay.com and do a search for 'behind the ear hearing aid' you will
get some hits for behind-the-ear models (most are newer one size fits
all models).

Here is one link to a Siemens behind-the-ear model, without the custom
fit earpiece:

http://cgi.ebay.com/ws/eBayISAPI.dllViewItem&category=75062&item=5584286694&rd=1