Hi, Benay148 -- I would seriously consider BidStart, especially for the area you're looking for. There are both full-time dealers on there, and also small hobbyist sellers who sell their duplicates, and others in between. Prices are pretty competitive, because there are so many buyers and sellers of US stamps.
To me, one of the advantages of buying there, instead of ordering off a list or from a catalog, is that you can usually see the scan of the actual stamp you'll be getting. (Some sellers just have a representative scan, but many (most?) sellers have scanned in the actual stamp for sale.) That way you can evaluate centering, perforations, and, if it's used, the cancel.
You can also create a wantlist on there, so that you get an email anytime someone lists a stamp that's on your list.
Most sellers are in the US, and most try to keep the shipping to a reasonable charge. There are auctions there, but a lot of fixed-price store items, too.
Here's a link to the stamps section of Bidstart:
and here's a link just to the US stamps section:
http://stampwants-stamps.bidstart.c.../2705/c.html It says there are 995,000 listings in the US category alone. I see that there are 4 sub-categories just for the area you've mentioned:
-- US 1901-1940 unused
-- US 1901-1940 used
-- US 1941-present unused
-- US 1941-present used
Hope that's helpful!
-- Dave