We bought ours online drect through Disney - they appeared to be much cheaper than those offered by travel agents or sold at the gate.
This is especially true at the moment as there are very good deals such as adults at kids prices, or kids free deals online at Disney.
Being Australian I can't access these, but there are apparently good deals for AAembers and Southern California residents if you can get someone to purchase tickets for you (tickets don't come with any ID, apart from annual passes, so anyone can use them once they are purchased)