Looks like you can construct the link to the NDK that you want and download it from dl.google.com:
Linux example:
http://dl.google.com/android/ndk/android-ndk-r9b-linux-x86.tar.bz2
http://dl.google.com/android/ndk/android-ndk-r9b-linux-x86_64.tar.bz2
OS X example:
http://dl.google.com/android/ndk/android-ndk-r9b-darwin-x86.tar.bz2
http://dl.google.com/android/ndk/android-ndk-r9b-darwin-x86_64.tar.bz2
Windows example:
http://dl.google.com/android/ndk/android-ndk-r9b-windows.zip
Extensions up to r10b:
.tar.bz2 for linux / os x and .zip for windows.
Since r10c the extensions have changed to:
.bin for linux / os x and .exe for windows
Since r11:
.zip for linux and OS X as well, a new URL base, and no 32 bit versions for OS X and linux.
https://dl.google.com/android/repository/android-ndk-r11-linux-x86_64.zip
