Go on eBay and search "4 lug mustang", you will find alot of aluminum rims. I would think that the 5 spoke COBRA rims would be the lightest aluminum rims you could get, but they are not "cheap".
You might also look for the old "ten-hole" rims, they are aluminum. I saw a nice one in the boneyard yesterday, in the hatch area of a pretty dead 'stang.