I'm not sure what other members might say, but if you want to just try an online stamp dealer, I've had good luck (long before joining this board) just 'hunt and pecking' stamps, $15-25 an order, from Golden Valley Stamps in Minnesota. They have a regular type website for just US, broke down by year, and sell year sets if desired. I was happy with what I got from them. You can certainly get cheaper if you look around, but even if you don't order from them, they show pretty much every stamp and have a price, so you could look around and see what you might want to look for elsewhere. Their prices seem to be right around the guide price, which isn't really fantastic, but it's not near the price of the rip-off dealers like Mystic and such..
If you want to look at their US stuff, you can try this link.
http://www.minnstamp.com/catalog/in...php?cPath=36that'll take you to the US page.
I'm sure if anybody has any other opinions of Golden Valley, they'll chime in. Like I said, I've used them to fill in holes in my later years (replacing stamps ruined by glazing), and for the non-valuable 'normal' stamps, they were priced ok and had them in stock, and I always had good results :)