Deezer is great if you are looking for specific recordings of classical pieces.
Although saying that they are some semi-official release from rock groups bands
For example Deezer has Midnight Oil has 1984 interview album, Unplugged Gig, Badlands 1985 (Live At Glastonbury) that are not considered official release of the band.
Beach Boys there are some non-official live bootlegs such as their concert 4th July 1981 which is interesting as there is a cover of Back In The USSR.
There are also some digital only release that are different to find in lossless format (Such as Beach Boys Live Sunshine)
Just download this program
Once installing it you can browse the database and see if there is anything you find interesting.
If you want to download something you will have to create a free account. If you want to download FLAC then you will have to pay for the Hi-Res account