An2net,
I'd exchange the money when the exchange rate is good and take it over with you to buy on the spot. You could try googling Cleaver Brothers and sending them an email to see what the best price they would offer the tix for.
Cleaver Brothers are on Harbour Boulevard in Anaheim; so they could be a convenient stop if you are staying in Anaheim. I've bought discount tix from them at least twice over the counter without any issues.....try sending them an email for a quote. Needless to say, the more days you have at the park, the better the discount. In general, you could save about $10-15 per adult ticket for 3 days ticket or longer.
Aside from everything else that has been mentioned, you'll also be paying for food in the park.