I would figure out which state the tolls are going to be highest in. Most likely PA if you are traveling the full length. And get the EZ Pass from that state. That way you are most likely to get the biggest discount for using it.
In PA you can buy them at the rest area's or even Walmart. However, there is a catch, at least with the Walmart ones. You must register it online and it can take 24 hours for it to be activated. I would guess the ones you buy at the rest areas would be the same.
Ordering one ahead of time should be the best option.
PA Ez-Pass