Sean, where are you located? You can get lab blocks from almost any feed/grain store. I live in southern California and use Ace Hi (High Fat) milled by Star Milling. If you live on the east coast Agway has their RMH line. I believe that RHM1000 is the one I used for mice but it has been 5 years now so I might have the number wrong. I used RMH exclusively for 10 years. I have also used and can recommend Mazuri Rodent Breeder, though this one is much more expensive. If you are production breeding I would suggest using the Rodent Breeder 11F. Lastly, is Purina Mills Lab Diet 5015 Mouse Diet.
I have personally used all of the above diets with good results. There is one other one I can suggest. I have never used this one myself but know breeders that do. It is Harlan Teklad Global Rodent Diet. For producing mice I would go with the 19% Protein diet. This is produced by Harlan one of the largest and best known Lab Animal breeding facilities in the world.
Hope this helps,
Jeff C.