As far as I'm aware, there are 3 Kosher stands at Yankee Stadium. They're located:
Field Level - Section 110
Mezzanine - Which section?
Upper Deck - Which section?

Food Available
Hot Dogs - $5.50
Wings - $5

Hot dogs are OK. They're not terrible and rubbery, and they come out hot. But they're boiled, not grilled. They're not thin nothings, but they're by no means a large hot dog or sausage.
Bring your own food, but if you're craving a hot dog, smelling all the non-Kosher options, than this can work.

Legends Suites
Legends Suites are the sections 14A through 27A, closest to the field - from 1st base to 3rd base. These are several hundred dollars, and are the absolute best view. They come with free food. There is Kosher options as well.