Look for '70's -'80's B-body cars, That was pretty much the source for my 15" steelies. They came in 15 x 6 and 15 x 7. I was fortunate to find the 15 x 7's.Now to continue the search for 15 inch steel rims
There's a code stamped in near the valve opening. Make sure they're 4.75 x 5 bolt pattern. Some B-body models are a 5 x 5 bolt pattern. It might be the later years. . Here's a list of vehicles to look out for.
'76-'90 Chevy Caprices
'82-'86 Pontiac Parisennes
'76-'81 Buick Century
'82-'85 Buick Estate wagon
The wheel codes are AE, FN, and TJ.
This was information courtesy of Turna some years back.