Getting NSS and NSPRIf at all possible, use your distribution's packaging of the Mozilla development libraries. For example, on Red Hat Linux, install the mozilla-nss-devel and mozilla-nspr-devel RPMs. Hopefully, those packages (or maybe the main Mozilla package) will install a "mozilla-nss.pc" pkgconfig file. That is how the Pidgin-Encryption install process will find those libraries.
If you must install the libraries by hand, they are at ftp://ftp.mozilla.org/pub/mozilla.org/nspr/releases, and ftp://ftp.mozilla.org/pub/mozilla.org/security/nss/releases. You will probably need to use the --with-nspr-includes, --with-nspr-libs, --with-nss-includes, and --with-nss-libs options to ./configure.
For some instructions about installing NSS/NSPR as part of compiling Pidgin on SuSE, see these instructions. They may be helpful on other distributions that don't provide devel header packages as well.