where do you buy 5 gallon buckets of soda?! That sounds like useful info for me to have anyway.
Buckets won't work, you'll lose the carbonation!
5 gallon steel kegs were formerly used in the beverage industry to distribute soda to restaurants, but not so much any more. When the beverage industry started phasing them out all those old kegs got dumped onto the secondhand market. Homebrewers have re-purposed them for storing and serving beer since 5 gallons is a much more manageable batch size than a commercial beer keg, and they are also much easier to clean and maintain without special equipment.
You could make your own soda and put it in 5 gallon kegs... a lot of homebrewers do that for the non-beer-drinkers in their family.
If it's a keg, try a liquor store that will sell kegs of soda. Really. I had to get a keg of root beer for an event a couple years back and they got me a full size one within a couple days.
Yeah, I suppose you may also be able to buy soda in quarter-barrels (7.75 gallons). But AFAIK the old 5 gallon soda kegs have become pretty rare outside of the homebrew and brewpub scene (some brewpubs will let you bring in a 5 gallon soda keg and fill it for you).