As the previous poster said, there is an outlet outside. We did just what you are thinking about - rope lights on our deck. I absolutely loved it and definitely plan to shove those lights into the suitcase again for our Thanksgiving trip.
I got mine at either the Home Depo or Lowe's - I don't remember which one but I bet they both have them. Call whichever one is more convenient to you and see if they have them. I bet you'll get lucky.