Jewel, Cub, Dominick's, etc, should all have it.
I never found the bottled version nearly as good as served off the tap at the restaurant/bar.
For what it's worth, the Costco on Clybourn has Goose Island root beer, 24 12-oz bottles, for $12.69. The best deal for the berghoff root beer is the $1 32-oz bottles.
Neither root beer is superb, but both whomp Barq's.