Frank Calia used to offer C.oleifera from his nursery in Cincinnati, but I
can't find his website any longer, and haven't heard from him in a year or
two (I used to supply him with P.trifoliata seeds).
C.oleifera is one of the varieties that was used in the breeding programs
at the National Arboretum, and by Dr. Ackerman, in development of most of
the 'cold-hardy' camellias.
A Google search for C.oleifera, cold-hardy camellia, Ackerman hybrids,
etc., should bring up a number of hits.
Here's one C.oleifera factsheet, to start out with. Jim, it looks like
Menomonie is way outside the potential survival zone, though.