You can only purchase from the iTunes Store in AAC. However you can easily convert them to MP3 in iTunes (though you may lose some quality in doing so) provided that they are not protected (i.e. they say 'Purchased AAC...' not 'Protected AAC...' - recent purchases in the Store are not protected..
Firstly, go to iTunes Preferences>General and click the 'Import Settings' button. Set 'Import Using' to 'MP3 Encoder' and use the next drop-down menu to choose your settings - you can choose 'Custom' and set the bitrate as suits you.
Click OK, then OK again to close the Preferences.
Go to the Music list; select the files you want to convert and from the 'File' menu choose 'Create new version' then 'Create MP3 version' (iTunes 11 - in earlier versions choose 'Create MP3 version' from the 'Advanced' menu).
MP3 versions will be created and placed in the music list (the AAC versions will remain there). You can drag them out if you want to save them elsewhere (doing this will not remove them from the Library). You can select and delete any files as required in the Music list. (Never tamper with the actual library in the Finder).