there's 4 airports (well, 5, but I don't count Burbank because it's universally awful to get to and from). LAX-- 35 miles, cheap to fly into, transfers can be expensive (shuttle bus, taxi or
Disneyland Express run by Grayline). SNA-- closest, 12 miles, very nice, small, but usually the most expensive, often by more than $100. Cabs are $45 flat rate, shuttle is a cheap alternative, DL Express runs here, too. Long Beach (LGB) not too far, 15 miles, often as cheap or cheaper than LAX, but fewer flights so your times are limited. Cabs are $45 flat fee also, but the shuttle costs more than from SNA, and DL Express doesn't run here. Ontario is 35 miles out, often cheaper, but kind of a pain to get to and from. Cabs and shuttles are more expensive.
That's just off the top of my head. Hope that helps!