You can order them from B3 or northern brewer. You are better off to try to find a LHBS that you can buy from. Even if you have to drive a few hours it's cheaper that paying shipping on 300lbs of grain. I drive to Akron, about 2 hours away, a couple times a year and stock up. Then if there is other stuff that comes up from time to time I can order it online.
What you get is kinda of dependant on what you brew. I always buy a 50lb bag of American 2-row, usually Briess, and a bag of Crisp Maris Otter. Those are the two base grains that I use a lot of. Then I'll get 5 or 10lbs of a bunch of the specialty grains.
I store mine in rubbermaid containers. Here are my specialty grains...

Also you can buy a grain mill from a homebrew supply of get a corona mill. I would like to get a nice roller mill, but haven't had the money so I got a $10 corona mill.






Pretty much you buy grain, store grain, crush grain. That is all.
Oh and you don't really want to let it freeze and thaw. Possible problems with moisture condesation and mold.
Travis