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McMaster-Carr is also a good place for screws (around $3-4.00 per 100): http://www.mcmaster.com/#
Give yourself plenty of time as they have literally thousands to choose from!
There's also Small Parts: http://www.smallparts.com/
Haven't looked at NWSL for screws but their prices are probably the same as most hobby shops - 10 screws for $3.00 vs 100 screws for around $3.00 from McMaster-Carr or Small Parts