Almost any grocery store. It comes in a plastic container. Arugula keeps better than spring mix but it will still pay you to check the expiration date on the container. Don't buy it unless the sell-by date is at least 4 or 5 days off.
Get a length of paper towels (maybe 3), open the container and wrap the paper towels around the arugula before re-closing. Helps keep the moisture away. Every time you take out some arugula, remove any wet pieces and re-wrap in the paper towels.