Harlan Teklad/Native Earth has some lab block you can generally get in large amounts for good prices. o: Not sure about shipping, as I have yet to order, but pet360 has a 40lb bag for a little over $36 which is insanely good. I rarely hear anything bad about the brand, as it's mainly used by labs and they naturally need to have quality so they don't get weird stuff happening to their experiments.
Oxbow has some REALLY good food for rats, no corn at all, but the problem is it's massively expensive. I hear rumors there are 40lb bags you can find online, but I believe the price alone was ~50-60 bucks and I shudder to think of shipping! On top of this, some people report their rats don't like it. I heard there was awareness of a palatability issue but I'm unsure how 100% true it is, as all of it came from forums.
Good luck on your hunt, I hope your ratties get better!