I'm probably repeating here, but I asked the brewer at a local microbrew (Artic Craftbrew in Colorado Springs) and he had no problem ordering an extra 50# bag and sold it to me at cost. (you just have to make sure to buy a couple of pints for his efforts!)
I also bought a full bag from my LHBS. (Long story, but I ordered one, it came late, sop I bought from the other guy, but I felt obligated to buy the bag when it did come in...)
My advice is to stick to buying a big bag of base-malt, like American 2-row barely, English 2-row or pale malt. Buy any specialty grains from the LHBS when you brew.
My first all-grain was just very basic beer: 8.5lbs barely malt, water, hops and yeast. Nuthin' special and nothing I would get upset about if it went bad. Turned out pretty good, and I've brewed that several times. (Added 0.5 lbs of wheat malt for head retention, played around with hops and yeasts.)
BUT, be sure to store the grain in a cool, dry place. I wouldn't recommend keeping it in the grain bag once its opened. I put the grain in food grade plastic buckets with TIGHT FITTING lids. (Get them from the bakery; cake frosting comes in those. I used extra buckets to build my mash/lauter and sparge tun.)
Also (jeesh, will he shut-up already?!?!) resist the urge to mill all the grain at once. Its ends up drying out. I did that and it may have affected the efficiency of the mash. I now keep all my grain, unground, and take a bucket with me to the LHBS, buy my hops, yeast and any specialty grains, and have them weight out what I need and mill it.!!!