pretty sure that Guest Relations do it at all 4 main parks but the rate is prob not good. What about doing it at the Airport, either at home or at MCO? You might get a better rate there.
You're better off to just leave your CDN$ at home. Exchange a small amount to US$ cash before you leave and then if you need more, go to an ATM and withdraw US$. There's a bit of a service fee but it's convenient and you'll get the same exchange rate that you're bank is offering at home.