The best way to get high quality files is to rip the files from CD into a lossless format, such as FLAC or ALAC (Apple Lossless).
320 kbps MP3 files are lossy files, and are, in principle, inferior to lossless files with respect to sound quality.
The sound quality of your files cannot be improved. That is, once a file has been encoded in a lossy format, e.g., 320 kbps MP3, that is the best that file will ever sound. By converting to lossy formats, certain information is discarded, and that information cannot be recovered, except from the original lossless file.
I believe Amazon and Google sell high bit rate MP3 files. Apple's iTunes Store sells high bit rate MP4 files.