As indicated clearly in the other answers, what you're asking for would be unsafe. Repositories allow you to not only get software, but to regularly get updates to the software to fix security holes and bugs. What you ask for would be like googling the whole web to find a package and blindly downloading it and installing it. That's a sure way to not only risk getting malware, but, even for good packages, to leave yourself open to security issues that are found later and updated in the ppa, which you would not be tracking.
So you do need to search to find a ppa, using the tools mentioned in the other answers. You should then personally vet the reputation of the ppa, since it could easily take over your whole system if you add it.
If you find a ppa that you trust, use add-apt-repository as described elsewhere.
But another aspect of your question is how to get just a set of specified packages from it. For that, see the info at How do I cherry pick packages from a PPA? - Ask Ubuntu. If you want an easier solution, you should file an enhancement bug request on launchpad with a good use case.