ITunes, for reasons of copyright, the music is sellable staff AAC and not ALAC more from few years. 7digital there is stuff all over the world, INCLUDING singles, at least in two versions: MP3 and FLAC. Knowing I would come to convert from MP3 / AAC to FLAC with quality imporvement and what software could I use? I Have had: dBpoweramp and Audacity. If I use Audacity, what filters?
MP3 is a lossy form of compression. Audio data is compressed or lost to adhere to the target bitrate. Once it is encoded as MP3, you can't get that data back. There's really no point in converting MP3 to FLAC, because the data is already lost. You aren't going to see any remarkable quality improvement just because it is FLAC. You will only end up making a bigger file that doesn't sound any better.
FLAC a "lossless" form of compression (or near enough as not to matter to the average person). You would want to make FLAC files from the original audio source, to retain as much of the original dynamic range and detail as possible. You can convert FLAC to MP3 (say for device compatibility), because it would essentially be like creating an MP3 from the original source.
If you are given the choice between getting a song in MP3 or FLAC and quality is a concern for you, then just get the FLAC version.
The only way to get free music from that site would be if they make albums available for free download, probably from a promotion. Barring that, no. Discussing ways to get for-pay stuff for free is beyond the scope of this site. That's not what it's for.
Compression is one-way. You cannot convert MP3 to FLAC and somehow magically restore the quality.