Purchasing Duck Stamps From The USPS

This may be a foolish question, but here goes: I have noticed that when I purchase duck stamps from the USPS by mail, they always send me the "souvenir sheet" version of the stamp, and not the single. (For example, Scott stamp 93A, and not 93.) There doesn't seem to be an option to purchase the single in the USPS catalog.
What is the simplest way to purchase singles of current duck stamps at face value? Can I get them at the local post office?
Thanks!

I haven't seen a response, but I figured out the answer on my own. It seems that the best source for purchasing duck stamps is duckstamp.com, rather than the USPS philatelic catalogue. The duckstamp.com web page sells both versions of each stamp, as well as Junior Duck Stamps, first day of issue covers, and other collectible items. I hope this helpful to other (new) collectors!