Ebay, swap meets or wrecking yards. The original wheels were non-safety bead style with a taper to the rear side of the drop center portion of the rim. All the aftermarket rims use disc brake type rims which have safety beads and more of a cylinder shape to the back of the rim, to clear disc brake calipers. The safety bead is an indentation that shows in the face (and rear ledge) of the rim that holds the tire bead on the rim in the event of a flat. If you want to keep it original, new aftermarket wheels won't look the same.
