If you're buying mice/rats on a weekly basis, just make sure to choose a good pet store that has its animals in clean conditions and keeps them undercrowded and fed/watered. Any that have constant wetness under their tails, scabbiness or red bloodlike substances dripping from their eyes, should be avoided; they are unhealthy and thus are not the best choice, as some BPs will go off feed at various times, and with each feeding you want to pack them as full of nutrients as you can!