You can try a local hobby shop or craft store. K&S is a common supplier here in the states. Ebay and Amazon are also good sources. Do a search for K&S tubing. This company also sells aluminum, and steel, but one is to soft and the latter is tough to cut. Brass seems to be a good middle ground.
Attached is a picture of the steel wheels for my Taigen Sherman. Notice the use of brass bushings. That's the pile on the left. You will mimck the same setup. Measure the shaft's diameter and the wheel hole diameter. These measurements will represent the inner and outer diameter of the bushing respectively. Now look for similar sized tubes. You will more than likely need two or three different sizes. You will slip these different sized tubes into each other to get close to the correct inner and outer diameters built up.
