What are the specifications for a GA-90C choke and where can I
purchase one in Toronto?

For an EB-2. Contrary to popular belief, virtually all parts can be
substituted with no apparent sound change. If you have one in hand and
you want another, measuring the characteristics and copying with
similar materials will work just as well. Even if it is encapsulated,
you can measure the DC resistance and inductance -- some cheap
multimeters have inductance measurement. Some (if not all) chokes were
humbucking, iron laminate cored, but it ain?t going to be too
sophisticated.

Kent Armstrong used to supply (for the Jazz Bass I think). Drop him a
mail. Whatever though, he should be able to advise.

http://www.kentarmstrong.com/index.htm

Best