Is there any special reason why you need to convert these files?
Re-encoding something already in a lossy format into a different lossy format will only lead to further sound degradation. Most players that can decode mp3 should also be able to handle mp2, so unless you have something really limited they're unlikely to be a problem as-is.
If you really need mp3 for some reason, then if possible you'd be better off going back to the original, lossless sources and encoding directly from them.
By the way, this is a very commonly asked question, and even a cursory search of the forum or the net should've turned up many solutions. Check out the "Similar Threads" box at the bottom of the page, for example.