On purchased ammo:
The best deal going is the Greek surplus 30-06 ammo as sold through CMP:
There are some requirement to buy from them so read all of their page.
The Greek brass is excellent so you would be building your supply of fired brass while shooting ammo intended for the M1 Garand.
If you buy any old surplus ammo be aware of a few things: Is it corrosive; Was it made for the M1 Garand: Does it use reloadable boxer primers or un-reloadable Berdan primers.
On reloading for the M1 Garand:
Todays manuals often have special section for reloading the M1 Garand which use powder best in burn rate for the M1 gas system. Using the wrong powder can hurt the gun (bent operating rod). Don't use a reloading recipe for bolt guns unless it is the same as the specific M1 Garand recipe. Stick to the listed M1 Garand bullet weights too.
If you buy new brass buy only a name brand, like Remington or Winchester or Federal. Have your buddies, that don't reload, save up their commercial empties. Stay away from Brazilian brass as it tends to be thinner and that upsets the neck tension holding the bullet into the case. The best once fired brass you can buy is Lake City (US government) but the primers are crimped or staked in and removing the crimp after removal of the primer is tough. The easiest way is to just buy new bulk Remington or Winchester brass from a search of the Internet for the best deal (Midway is usually NOT the best deal but usually has it).