Can i buy a ticket inside the stadium?

Outside the stadium

No. Typically to get inside, you'd need to purchase a ticket prior to the event. However as stated by Steve Bender, there is a will call area inside to pick up tickets and leads me to believe you can purchase new tickets as well. Check the Yankees Stadium website for further information.

Well actually they have an area inside the building which is only accessable by the outside entrance you can purchase tickets there its a will call area where people also get tickets they've ordered online or had left for them.

No

Outside