Hey, P.,
Not sure if you're not going to have a chance to get to a store or not before you go, but when we flew with our twin daughters when they were around this age, we used stickers and new lift-a-flap books. Also, if you have a little bag or purse to put small toys in that they can take out and put back in, it might not matter if the toys aren't new. You could search at the bottom of the toy chest for some old favorites they haven't played with in a while. We also found that having a lot of snacks helped.
good luck!
A.
p.s. Are you flying with another adult? If so, I don't know if the girls will be sitting in your laps or have their own seats, but if they are in your laps, know in advance that the airline is unlikely to let all four of you sit in the same group of seats. That's because there are usually only four oxygen masks above each group of three seats. So if there's you and your partner, each with a toddler, and another adult in the third seat, that's five people and only four masks. We always had to sit across the aisle from each other (and sometimes it mattered which aisle, too, because on some airlines, only certain aisles have infant life vests).
This is something to keep in mind if you decide to bring a portable DVD player, since the girls may not be sitting together and able to both watch at the same time.