L. R. Du Broff wrote:

Does anyone know where I can find a four-foot 50-ohm coax (I believe it's
R17x/U, where x is either 4 or 9, I can't remember) with a BNC on one end
and a 3.5 mm mail on the other end?

Make one.

All the parts should be available online or in a truly comprehensive
computer/electronics store. Or Matronics online... I live in Houston and
there are both a mom and pop computer place that have the parts, as well
as "Fry's Electronics" which is an electronics/appliance superstore that
is a nationwide chain.

It's not rocket science to make a cable. Usually instructions are on the
connector packages.

Dave