I can’t help you with New York, but at Dodger Stadium, the hot dogs were first provided by John Morrell & Company, which had a meat packing facility in Los Angeles, and later by Clougherty Packing, whose products were sold under the brand name Farmer John. I believe their facility was in Vernon or thereabouts. This company later merged with Hormel, which still uses the Farmer John brand name and still makes the Dodger Dogs you can get at Dodger Stadium.
At Riverfront Stadium in Cincinnati, we used to enjoy Kahn’s Hot dogs while rooting for the Big Red Machine. Kahn’s... “the wiener the world awaited”