Where can you buy music CD's?

For prerecorded music albums on CD, I currently lean towards Amazon because many of the albums on CD will be auto loaded into your Amazon Music account when the CD is shipped.
I also get some from Kickstater and IndieGoGo from some of the indie artists I follow. I know some have work of CD Baby
Sometimes for the band's website or their bandcamp
My local library has a book sale ten times a year that includes used CDs sometimes I find them at Salvation Army or other thrift or second hand stores.

Stores specifically selling CDs, some department stores, companies on the internet specifically selling CDs, artists' websites, lots of places.

If you're talking about discs that already have music on them, then go to a store like Target, Walmart, etc and look for an artist you like and buy the CD. Not really difficult

If you're talking about blank discs to burn music onto, again, stores like Walmart and Target carry them. Make sure you get CD-R discs and not DVDs. You can burn music to DVD discs, but no CD player would be able to read them as they only read CDs not DVDs.