The biggest thing to grasp is that the 1911 is not an AR-15 or Glock. You usually just don't buy another part, take the old one out and put the new one in.
There is such a large variety of 1911 designs that you often need a gunsmith to fit the part. If your intention is to have a gunsmith do the work, I'd check with the gunsmith you've picked and have him/her make a recommendation.
If you are planning to do the work yourself, you most likely would be better off buying a pistol put together the way you want it from the factory. Unless of course you just like to tinker with stuff and won't be too put out if you somehow ruin a $500+ pistol trying to fit some part.