My sister buys all her grains for baking bread in large quantities like that. She stores them in large 5 gallon buckets in the basement and hasn't had any problems. Her basement is a bit damp/musty. Can't you buy empty 5 gallon buckets from Home Depot, Lowe's or even a rest. supply store? You could save on shipping.
I guess my question would be - the buckets at someplace like Home Depot aren't going to be food safe right???
I think a 5 gallon bucket is a 5 gallon bucket. Someone feel free to correct me. Do you have a local bakery in your area? You could ask them if you could have their empty 5 gallon buckets. I think frosting and filling comes in them.
There is a difference between a Home Depot bucket and a food safe bucket. Especially for storing, I would try the food safe route. You might try checking for a resturant supply store in your area. I also think calling a bakery for a used frosting buckets is a good idea. A friend of mine did that. They we're free but she had to wash out the old frosting.