I'd second what Rich said about it being "hot" spots from them itching, breaking the skin and then either continuing to scratch at it and/or the other mice picking at it. As far as Kent Feeds, here's there web-site for finding a distributor:

http://www.kentfeeds.com/

I feed my rats Native Earth which is made by (and is) Harlan rodent food. I've never had any issues with "hot" spots or breakouts like what you have pictured there. I don't have a huge operation or number of rats but my litters, on that diet, average 13 per female, with the highest being 18. That's all I've ever fed and I'm perfectly happy with it. There's any number of options, some better than others and it all depends on what you want to pay and where you're located. Good luck and hope that helps!