Online, ordering is OK, but frankly you are going to be getting tiny trees and shrubs for the most part that way.
As for "Distributors" - do you mean wholesale? Wholesale in Atlanta is strickly wholesale - you must have a business number and license related to landscaping to purchase.
There are TONS of places to buy plants in the city. The yellow pages is choc full of nurseries, home depot, lowes...you can get a Farmers and Consumers market bulletin if you'd like to get some fair deals (often means a good drive to find the deals - not necessarily right in the Atlanta area).
In Loganville the Growers Outlet has good deals. They have limited varieties at any one given time, and usually the smaller scale shrubs. Rarely any trees.

At any given place you can find good deals. Buck Jones website has their specials posted (usually trees and shrubs), and many places have great sales in off season times (closer to winter) but you get limited choices then if there is something particular you are looking for.