Aldi USA generally doesn’t stock any sort of sparkling water. Many big full-line supermarkets do, but that’s not always convenient when driving or bicycling.
McDonald’s can nearly always be counted on to have a fountain that dispenses soda, though you generally have to slowly explain the concept to the counter help—unless he or she is Mexican and therefore familiar with agua mineral con gas. Many Burger Kings have fountains that are supposed to dispense plain soda water, but often they’re broken. Sbarro is another good bet; Taco Bell never ever.