Awww, someone is probably very sad they lost him/her! Our friend lost her budgie and she was desparately hoping someone like you would find him and return him, but he never came home and she was devastated.

So I'd definitely post some 'found' flyers (just say 'bird' or 'budgie' don't give specifics so it will help to find the correct owners), post an ad in the newspaper, contact local vets/shelters and post on the links that Carrie provided above.
Our budiges are given regular budgie seed, but we also provide them with Zupreem pellets and they actually choose to eat the pellets some days over the seeds.

They are very high energy birds so they need more fat than the Quakers do. Ours also like greens like broccoli, romaine lettuce, kale etc. Definitely offer him/her some millet as its a huge favourite and s/he'll most likely dig in right away... if s/he's been out for a while s/he's probably very hungry. Our budgies are so vocal, I love listening to their little chirps all day long!

Like the others have already mentioned quarantine is very important when it comes to unknown birds, especially one that's been living outside.
Good luck, hopefully you can find the owners!