Surprisingly, yes. Most listeners and blind tests have proven that 256 kbps AAC is transparent (indistinguishable from the source CD) for the vast majority of ears. Because AAC is a more efficient algorithm than MP3, a 256kbps AAC file often retains more high-frequency data (cymbals, hi-hats) than a 320kbps MP3, resulting in a smaller file size with better sound quality.
Bandcamp is the king of direct-to-fan sales. However, their default download format is MP3 V0, FLAC, or WAV. Itunes Plus Aac M4a Sites
If you are searching for "iTunes Plus AAC M4A sites" because you want high quality but don't care about "owning" the file, consider these: Surprisingly, yes