if you add on to a pkg, when you first book it, you have 10 days to cancel, You can ADD insurance on a pkg up until final payment. (thats what they told me just today)
You can get it outside Disney (i have used insuremytrip.com) several times and they were great, you can get MANY quotes, different carriers, and compare prices.
As an aside, the riders on the policies or Exclusions are REALLY what matter. ie, pre-existing condition waiver means, if your family member HAD that prob or treated with it, even a LONG time ago and you chose NOT to get the pre -existent waiver, you do NOT collect if you need to cancel.
You really need to familiarize yourself with the policy, some have "dates" like last 90 days and some say nothing, then exclude if they treated in the last 150 days.
Some carriers will only let you ADD pre-existant waiver if you do it within a certain period of time (like within 30 days of original booking) so, it really pays to ASK if you have ANY q's at ALL!
By the way, I go in august a lot and never had to use it, but have bought it. Had a family member that had to cancel and THANKFULLY we had the insurance and got back thousands which otherwise would have been lost!
Good Luck!
So, my only HINT is, READ and ASK specifics if you have ANY q's!