You can buy seed anywhere. You'll need a spreader as well, unless you intend on hand bombing it all over your yard (not recommended). It'll take a few years to grow in well, and when I seeded in the past I kept adding seed twice a year to cover up the bare spots. It's not that hard as long as you stay off it and keep it watered.
More detailed instructions including tilling and seed selections are
here.
I went with Kentucky Bluegrass, but only because it was the same type they use on golf courses. I think it's supposed to be more durable as well??