They certainly sell them online.
https://www.walmart.com/ip/D12-World/32061444
Related items show a bunch more.
The music industry does not employ a 'rating system' like the film, TV, and video game industries. Rather, it's a catch-all for any and all content that arbitrarily may be viewed as offensive to a broad demographic of potential customers. This takes the label from a mere rating system to more of a form of soft censorship.
I've seen albums that are largely instrumental labeled with the parental advisory sticker, because the sounds are intense and experimental. I've seen the sticker on albums whose only crime was being critical towards a religious institution. The label does a lot of harm, while I am not so sure how helpful it truly is for parents.
It's been done and over for almost 30 years. Whatever damage it did to the music industry and the culture, we've fully realized by now.
Laws that force fast food restaurants to put calories on fast food menus are annoying too, but we'll survive and I'll still get my burgers and fries.