I am looking to make a couple of tables that raise and lower. Each
would have a handle at one end, which when turned lifts or lowers the
top. The proposal is to use 2 screws linked to the handle shaft by a 2
pairs of miter gears.

I am having trouble finding an econmical source for suitable miter
gears. They do not need to be high quality or extremely large, but
need to accept/have up to a 3/4" diameter bore. They could even be a
strong plastic. Anyone have a suggestion where I could find something
suitable; preferrably in Canada?

Is it possible you may be able to use a worm and worm wheel? You
could make those on your lathe probably quicker than you can find the
proper miter gears you're looking for.
Ken