I'd start by avoiding feed stores. The wheat there, while cheaper,
tends to be intended for use as feed, not flour making. It tends to
not be cleaned as well, to have impurities, and not a very good
flavor (according to Laurel Robertson author of "Breads from Laurel's
Kitchen").
So, what to do?
Go to a health food store and ask to speak to whomever handles bulk
orders. They are happy to order large quantities - it helps them
meet their minimum orders from their suppliers. They usually
discount the purchase and you don't pay shipping.
Next, shop around online. Honeyville Grain ships any order for $4.49
anywhere in the USA. Sometimes their products are more expensive,
but the savings on shipping can change the picture.
Also, look around for food coops. They are usually very cost effective.
-Mike