You can get FLAC
here. HDTracks offers 96Hz/24bit recordings by audiophile labels like Chesky at $16 a pop, and if you're listening from a computer>DAC>Amp, you
will be blown away - the only problem being that "ordinary" cd's will sound limited forever afterwards.
As for the original question: in my opinion, if you can't tell whether a FLAC file is original or converted MP3 just by listening to it, then you don't need to worry about it in the first place. Sorry to be so blunt, but think about it... it's true, isn't it?