I've never applied to Disney, but have read my fair share of cover letters for people applying for teaching jobs.
Personally, I think that the cover letter is the secret weapon in the job search. It's the one thing that gives employers an idea of who you ARE, as opposed to what you've DONE. But far too many are dry and show absolutely no personality.
Its purpose is to leave the reader thinking "Now THAT'S someone I want to meet!!!" as opposed to rehashing your resume. It's not your chance to highlight why you want to work for Disney... they don't care that you've been a lifelong fan. What they care about is why they want you to work for them-- why you'll be a great employee. The two are NOT the same!
You want to come off as professional, but to let a little of your personality shine through.
So perhaps you've spent time acting as the mascot for your high school or college.... you can include a line about your dreams of moving up from a bird costume to one of a duck or mouse. Or perhaps you've worked at Chuckie Cheese, and can see the obvious parallel. Or perhaps you're a mom returning to the workforce, and have internalized the value of getting every detail just right for your kids, and would like to broaden those skills to the kids of other moms. (OK, not well phrased, but you get what I'm going for.)
Once you've written that first draft, be ruthless in your proofreading. There's no excuse for a single typo on either your cover letter or resume, and even one may just get your packet tossed into the circular file. (I've read cover letters from applicants who got the name of the school wrong. Really?? Can we expect the same lack of attention to detail once you've been hired??) Then walk away from it for a day or so, and look at it with fresh eyes. Give a copy of it to someone whose opinion you value-- your mom or dad, or college professor, or someone similar.
Do NOT download one. You don't want to be the third copy of that particular cover letter they read today. And, no, changing a few words doesn't make it a new cover letter. No matter how obscure the site, someone else has found it. Besides, a generic cover letter doesn't show who YOU are.
Good luck! Let us know how it goes!