In Audacity 2.1.2, I am trying to convert an mp3 song to WAV format and have a constant bitrate. Which choice do I use as I export my music file out of Audacity? WAV 32-bit float PCM or WAV 16 bit? When I select either of these, I don’t see a check box for CONSTANT BIT RATE like when I export my file as an MP3. Instead of using this MP3 and running it through Audacity, am I better off just to download lossless audio directly? Which Internet source(s) offer the largest library for lossless audio purchase?
WAV is a “perfect” sound format. It’s always at maximum bitrate and has very few options.
44100, 16-bit, Stereo is the sound format of an Audio CD—also the Audacity default format.
48000, 16-bit, Stereo is the sound format of non-surround video.
Audacity will be happy to convert an MP3 to WAV, but it will not make the MP3 compression sound and damage go away. WAV will reproduce the MP3 damage exactly correctly.
Once you MP3 something, you’re stuck with the damage, although you can make repeated MP3s from the show or music and make it much worse. That’s why MP3 is a terrible production format.
You should get your work as close to uncompressed as possible. Make the MP3 just before you send the work to your personal music player or on-line account where all you’re going to do is listen to it.
There is one oddity. If you read a book for ACX-AudioBook, they will insists on delivery in MP3. We suggest strongly you do it in the highest MP3 quality they will accept so when they do production, your voice doesn’t turn to trash.
Koz
Exactly what are you trying to accomplish? What’s the purpose of your WAV file?
Uncompressed audio is always constant bitrate and it’s easy to calculate:
i.e. Uncompressed CD audio is 44,100 samples per second, 16-bits, and two channels -
44,100 x 16 x 2 channels = 1411 kbps
Lossless compression (FLAC or ALAC) will give you a bitrate that’s a little higher than half the uncompressed original bitrate.
Instead of using this MP3 and running it through Audacity, am I better off just to download lossless audio directly?
I you want lossless, yes. If you de-compress an MP3 to WAV and play it, that’s no different than playing the MP3 and allowing your MP3 player to de-compress it as it plays.
But, it’s important to know that a high-quality (high bitrate) MP3 will often sound identical to the uncompressed original (in a proper, scientific, blind, level-matched, listening test).
Which Internet source(s) offer the largest library for lossless audio purchase?
Anyplace that sells CDs. 
Most music is not available as lossless downloads. If you look for lossless downloads you can find 'em, but if you are looking for a particular song/album/artist, etc., the odds are slim.