Ticket sources other than online distributors? Only in Florida at the box office. Getting in to the Cards and Braves games won't be a problem. You will be able to get tickets at the gate and the ballpark is small enough that there is hardly a bad seat. The Yanks game will be tougher, your two best choices are to pay premium prices on StubHub or to show up two hours early and look for someone with a couple extras for sale. At any given time on StubHub most or all of the tickets will be overpriced but people's travel plans change so if you check it a couple times a day there will be bargains to be had.
The Nats play the Astros in Kissimmee on the 13th. I highly recommend the hour car ride from Viera. The ballpark is very small and it is easy to get great seats behind the visitor dugout. Check out Gaterland while your in the area.
In fact, at Space Coast, walking up to the box office is the best way to GET tickets, talking to a human is the best way to convey your wishes, the online system is crap.
PB is correct, you won't have any problem with any of the games except the Yankees game.
PM me, as of now, I'm planning to be there for all the games you mention but things change.