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.

Perhaps, you should stop reading that other site.  How streaming sounds is dependent upon everything from the wall to your ears.

Qobuz has all the sound quality you need at present and for many equipment upgrades into the future.

Streaming is the way to go. Streaming makes up about 90% of record company revenues now. It’s easy, it’s cheap, it offers more music than you could collect on your own and it’s great for exploring new music.

Keep your cd player if you already have a cd collection or like to collect physical formats. .

Many people rave about the Schitt Yggdrasil DAC (yes that’s all spelled correctly) with Bluesound Node streamers. That or one of their cheaper DACs would probably be a solid upgrade for not too much money.  Just do a forum search for Schitt Yggdrasil for many opinions