Yep. All the parts to build one are available one at a time. But much like building an entire car piece by piece, it's cheaper in the long run to buy a crate......IF you can afford it.
As far as "best place to buy from" goes, that will come down to cost, location, and reputation. I personally wouldn't buy **** from Ray Barton because of the screwin he gave a friend of mine. But Anyone who sells M.P. parts will have what you need.
Most people who work for a living can't afford to buy restored cars, or crate motors.