yankees.com has plenty
ticketsnow.com has end zone bleacher seats for 25 bucks + fees
I ve used them a couple of times before........they screwed me once, for game 1 of the Yankee playoff game. I bought tickets at a price, then never reconfirmed.........because they were getting 3x what I paid for them. Worked out ok, that game got rained out and rescheduled so it worked out. They have plenty it seems, so you should be in good shape