Vendors are available on all levels of Busch Stadium. They'll typically carry typical baseball food (hot dogs, peanuts, soda, water) along with cool treats and beer. Though all beer sales will stop at the end of the 7th inning.
The 300 and 400 level seats share the same concourse known as the terrace level. You'll find restrooms, concessions and all other amenities found on the lower levels. The closest restroom to section 354 is located just up the stairs from those seats and away from home plate between sections 354 and 359.