Hi Jen
You don't have to get them exact - they can overlap - or fall a bit short
Finderscope - 3"
Eyepieces - they come in 2" and 1.25" eyepiece sizes, even though the eyepieces are all sorts of diameters.
Secondary mirror - you should get a specially designed heater - a normal band heater won't be much use.
I have the "Dew-not" - they seem to be excellent. Not sure if they are retailed in Australia?
Eric