What is the highest quality digital music file? Where can I get it?

Theoretically, the highest quality digital music is DSD 512. I doubt your current equipment can decode it (I am not familiar with the Bluesound Node). Qobuz is actually pretty good, same as Tidal. You can buy digital music on line at places like NatIveDSD and such. In my experience, local music sounds better than streaming in my system (source Melco server and LUMIN X1). 

>> I was reading a forum from another site suggesting that streaming was fine for background but not good enough for serious listening. <<

Bullshit, in my opinion. Either a preconceived notion or a statement by someone who heard a system where streaming wasn’t set up as well as the local sources. You know, whatever the viewpoint -- usually contrarian -- you can find it from some guy on some forum somewhere. The person or chatbot at the other end of a comment doesn’t necessarily have special expertise. (Including me, I suppose.)

>> Currently, I am set up to stream only using Qobuz through a Bluesound node. I have a CD player but no media at the moment. For quality of sound, am I better off with other sources? If so, where do I get them? <<

I have not tried a NODE. If you are using its own DAC, they are not universally well regarded. So a better external DAC might be helpful, depending on the DAC and the rest of your setup. Eventually, you might try a costly streamer like an Auralic or Lumin to see if you like it better.

Having compared Qobuz to CDs, local files, and such numerous times, I think it sounds just as good as they do. If you want to try local files yourself, you can buy 96/24 files from places like Presto Music. I tend to avoid HDTracks, as their remasters can be worse than the originals, IME. Better to download stuff that is new and not expect perfection from anything remastered for digital. I am not a fan of DSD for various reasons, but many are, so you could try that, too.

Thank you both for the feedback. The Node is a recent upgrade from an Andover Songbird and I am still using the Node's DAC although looking at upgrade options.

I'm just getting back into 2 channel home audio after a 20 year break. Lots of new technology to catch up on.