Nice choice, crossplaying as Tifa. She's probably my favourite female Final Fantasy character.
I remember, one time when I was dressing up, I had a strapless dress on that required me to have boobs so it would stay on better. Being the low-to-no budget person I am, I just used socks.
But here's an interesting thing you can also do. First, Tifa has big boobs. You already knew that. Anyhoo, get a bra that you think is close to her cup size. Find a circular object, hold it up near your pectoral, trace a small part of the circle with a brown eyeliner. Repeat this on the other side, using the same circular object you used before. Make sure the two marks aren't too far apart or too close together. Blend it out, and you should get a nice shape, almost like a Y shape. A good tip is to start small with the liner, because with cleavage, the lines should get darker as you go down.
As for filling the bra, I'm no expert. Usually I just use anything, socks, toilet paper, whatever. But maybe someone else here can help you with that. Good luck ^.^