The Corps of Engineers lakes/parks? I think there's an online process, but if you're close to Lanier at least, you can go down to the Project Office near Buford Dam, and they'll take care of you on site. I'm not sure if other Corps lakes have similar offices, but they may.
The PMO is only open on weekdays from 9-3, but when I called down there last week the nice lady mentioned you can buy them at the Corps ramps on the weekends. I'm going to try this tomorrow and will try to remember to report back if successful.
I dug a bit. Here is the contact info for GA Power's regional land use HQ's; IMO, the best thing to do would be to call them up and just ask if they can help.
North Georgia
North Georgia Land Management Office
4 Seed Lake Road
Lakemont, GA 30552
706-746-1450
Central Georgia
Lake Jackson Land Management Office
180 Dam Road
Jackson, GA 30233
404-954-4044
East Georgia
Lake Oconee/Sinclair Land Management Office
125 Wallace Dam Road
Eatonton, GA 31024
706-484-7500
West Georgia
Bartletts Ferry Land Management Office
1516 Bartletts Ferry Road
Fortson, GA 31808
706-322-0228