Most large home centers stock ext doors...and can order more exotic versions or fancy styles. You can also find them at a local millworks shop.
My company has new exterior doors here in metro Atlanta. We can also put glass in the one you have if it's in good condition (other than paint/stain).
Check with Builders Surplus off of Atlanta Road just inside the Perimeter. Be sure to have your measurements with you. As with all surplus companies., if you hesitate purchasing, it may not be there upon your return. Good Luck !