Hello!
There are so many sizes of wire, that you can always eventually find a good one. And they should'n be too hard to find, neither. What you want is a shop for electronics - Radio Shack or something the likes. Copper wire is best for details that are not meant to actually carry any serious weight (but you could still lift your T-34 model by the copper lifting rings if done right). For thicker wire the one used to wire houses is good, for thinner wire you can cut them off electronic components (like diodes or resistors), or buy wire used to wind coils (you can have them in diameters every 0,1 mm) - the silver coated wire is the best, tin coated will do too, in a pinch you can even use the lacquer coated wire (also found in electric motors and solenoid coils). Hope it helps, have a nice day
Paweł