If you set your fire up using the Minion method, enough for a full ring will be more than enough.
I also just go ahead and by the big bags. Like grantw said, it's no like it won't get used and most times you get at least a little break on the cost over the smaller bags.
Royal Oak is the best bang for your buck, in my opinion.
Although if you're feeling flush with cash, I'd recommend The Good One (if you can find it).
Does Colorado Springs have a 'real' BBQ store? Seems like they should since it's a big enough city.
Yep - call these guys and see if they can recommend a great lump charcoal for you: