iTunes allows you to burn purchased tracks to an audio CD-R. Just create a playlist and burn that to a CD with iTunes. At that point you have an audio CD you could make MP3s from, with iTunes itself or any other CD ripper.
NOTE: When you go from one codec (e.g. AAC from iTMS) to another (e.g. MP3) you will generally get "compression artifacts" which can degrade the quality of the file. Some people suggest that to avoid hearing this you should use a higher bitrate (e.g. 192 kbps) for the recompressed file, making the MP3 bigger than the downloaded AAC