Besides record stores, where can I buy CDs?

Target. Wal-mart. Online at Amazon, eBay or Discogs.

You can't buy them anywhere else. Sorry, that's it, nowhere else, not happening, not on Amazon, not on eBay, not at Walmart or Target, nope, not sold anywhere else, record stores only, illegal to sell anywhere else, don't try looking on your own, can't do that, don't ask a friend; ask only strangers on the internet, don't call anyone, don't shop anywhere else.

Before the pandemic, I bought a lot from the local library used book sales.

Amazon is nice because you sometimes get the files loaded to your Amazon music account if what you select is part of "AudioRip".

Best Buy no longer sells audio CDs but Target, Walmart, and Barnes & Nobel sell audio CDs.

Amazon and other such online sellers of such things exist, you know.

Also, if you're OK with used discs, try your local classified site, say, craigslist in the US and Kijiji in Canada.

I have bought most of my CDs new; 

others have been acquired at estate sales; 

some have come from a library "gift shop" or thrift shops.   

you could try amazon, they sell everything

Amazon, thrift stores, eBay, yard sales, online record stores.