I beg to differ with Duncan Brown. The purpose of
a high voltage loop supply is not to overcome loop
resistance but to build the current up quickly, in
spite of the selector magnet inductance, until it
reaches the value set by the current limiting
resistor - 20 or 60 mils.
Personally I like Ralph Mowery's suggestion - buy the
DC to DC converter on eBay. It is not too expensive and
should ship quickly from the U.S. Just mount it in a box
(for safety), set it for 120 Volts and add a wirewound pot
to adjust the current.
Do what Harold and I do - go to your local Lowe's or
Home Depot for a project box. I am attaching a picture of
my two portable (as opposed to rack-mount) loop supplies.
Tom N3CRK