Stock cop wheels were 15x7's. Nothing bigger. I've owned a dozen sets over the years.
If you've come across one that's 15x8 then it's a Super Coupe wheel. At a glance they look the same, except... the cop wheels have tabs off the hub for hubcaps. Super Coupe wheels are a clean circle with no hubcap holding tabs.
If you've found a set that are wider width, then they are, indeed, customs. I'd get 'em to a tire shop or local wheel shop and have 'em checked for straightness and make sure they can take a balance.
I know Stockton was making 16" cop wheels not too long ago and was thinking about going with them on my M-body build. I haven't checked recently to see what the availability is.