I've seen Coca Cola cars on eBay. I presume that you are talking about tank cars. I believe that LGB made them but generally those are two axle. On this side of the pond I'm generally looking for 4 axle cars. If I ever see a deal on a nice 4 axle Coca Cola tanker, I'd go for it.
Sadly, I've noticed that prices on G scale rolling stock have gone up noticeably in just this past year. It might have something to do with the changes in LGB ownership and the demise of Aristo-Craft, both great manufacturers.
To match the rising prices, and to add more sadness to the plight of model railroaders the world over, the cost of shipping has skyrocketed. I'm in California and numerous folks in New York are charging $40 USD to ship a single G Scale freight car across the country. Even if the price of the car is reasonable, the shipping is a non-starter.
Hope for better times. LOL