Left from your "Home" on the main GT Mode map. GT Auto area - Click
Wheel shop - click
Pick the tyre type you want the rims for - Click
Now at the bottom of the 3x3 matrix of wheel makers you get a thin line "Used Wheels" I missed this for a long time - click
Now you'll get a line of alloys to choose from, these cycle every day, and repeat every XX days.
You can either keep buying rims until a set you like comes up... this wastes days, but if you want the wheels then you'll probably do this.
In this area there are several rims to look out for.
The Jaguar E Type wire rims are probably the best but Green Honda NSX JGTC rims come up from time to time too.
Hope this helps,