If you are booking your flight separate from your resort, then you will need to get insurance for each of those. (I think you already realize this from your question.)
We always book separate and get the resort insurance when we book the resort and then get the flight insurance through the airlines when we book the flight.
For us it is worth it to get the flight insurance as it only cost about $100 more and when I've paid $1000 for tickets that are non-refundable, it is worth it and we've had to use it once.
It is also wise to buy the flight insurance as it will cover any traveling expense you might have due to flight delays and cancellations or lost luggage. Your resort insurance only covers incidents that occur while you are at the resort.
Long answer to a short question: we buy the flight insurance from the airline we book with.
Juliann & Jeff
Jamaica Soon Come