Zamzar.com is a good conversion site. It's fairly convenient. Won't allow protected files like m4p's but I don't think m4a's are a problem. You'll have to check.
You could go and download xillsoft audio coverter straight from the site I believe, just google it, its what I use for all my audio/video coversions
Try dbPowerAmp. Note that encoding from lossy format to lossy format is transcoding, and you will lose audio quality.