I use 45 gallon and 55 gallon steel drums. My dad used to work at a factory that used them and he cleaned some out and brought them home. The 45 gallon ones will hold two bags of feed, and the 55 gallon ones will hold three bags of feed if you leave the feed in the bags. Once I get a few more rabbits and can devote an entire drum to rabbit food, I'll be pouring the feed into the barrel. Out of the bag, the 45 gallon drum will hold 3 bags of feed, and the 55 gallon one will hold 4-5 depending on the size of what you are putting in it. If the grain is large/light (think oats or deer corn) they hold less.
If you have any factories around, call and ask if they ever have any available. They will usually re-use them until they cannot use them any more, but once they're done with them, they still make excellent storage. They're solid metal with metal lids, so no rodents. The few racoons around here haven't figured out how to get the lids off, so no problems there. The only problems that you may run into is the fact that they don't stack, and are extremely heavy when they are full. Do not try to move them once they are full.
Hope this helps,
Emily in NC