Try the internet Dr Shrink - see
Dr-Shrink.com - International Shrinkwrap Supplier , Wrap Anything!
Shrink wrap (material only) will cost you about the same as a tarp, probably more, but you only use it one year. That said, it does work better than tarps, especially in the NE where snow and ice are a problem. I used tarps for several years and always seemed to end up with large blocks of ice caught in the frame. I've used shrinkwrap for three years now and have had no problems (other than those of my own making).
In addition to the plastic material you'll also need tape and a heat gun. The heat gun is several hundred dollars and tape will run $30-40/year depending on the size of the boat and how much you want to use (I always use too much).