Firmware for AirPort devices is distributed from Apple in the form of basebinary files. AirPort Utility checks http://apsu.apple.com/version.xml for firmware updates; links to the available firmware downloads are available by browsing to that link.
The encrypted firmware files are downloaded to the following path,
The contents of these files is encrypted. Vince Cali wrote a nice utility that decrypts these files, and released it as part of the AirPyrt toolset. Note that you need to run decrypt twice to get the unencrypted image. Also, the image is a bootable NetBSD binary, with a RAM-FS section inside it containing crunchprog.