For some reason, the names of places elude me...for some reason, insuremytrip comes to mind, as does travelguard, but those might just be made up in my brain.
However, if you're doing everything separately, you might not even need it. Most hotels have a cancellation policy that is clear on how many days in advance they need to know you're not coming, and how many nights will be charged if you cancel too late. You would *at least* want to know if the premium is MORE than the amount of, for instance, one night's hotel fee if you cancel late.
You might be able to choose an airline that allows canceling with no problems, like Southwest.
And tickets, well, you'll want to use tickets for when your trip is re-planned, if you have to cancel, so you might as well just buy the tickets now, because the prices on tickets will only go up!
You could also wait to buy tickets until you absolutely know that you are going, then buy them online with either the will-call option or the e-ticket option (go for e-ticket!).
In short, if booking things separately, apart from the flight, I'm not sure you NEED to get
trip insurance. Just be fully aware of cancel policies of the various aspects of your stay.
And also make absolutely positively SURE that you understand what the insurance would and would NOT cover! In a recent discussion, someone was saying that the trip insurance they bought would not cover for pre-existing conditions within a certain amount of time. So if little Johnny got strep 6 months ago, and now he's gotten it again, the insurance is NOT going to help you.