What I've been doing is just grabbing the three files that make up the debian source package from packages.debian.org. So I search for package 'foo', go to its page and grab foo.tar.gz, foo.diff.gz and foo.dsc. unpack the tarfile, applly the patch, cd into the package directory and run dpkg-buildpackage. at least i think that's right. anyways, it should give you someplace to start.

-john