The nix package search website is OK, but it doesn't let you filter by the names of installed binaries. A lot of the time, you have a question like "what nixpkgs attribute do i install in order to get the `python3` command". I recently wrote a command line tool that allows you to do this. It uses the same elasticsearch index as the search website, but allows more powerful filtering. If anyone is thinking of getting into nix, please consider trying it out!
https://github.com/peterldowns/nix-search-cli