If you are just needing a small amount of plastic like that, you can get it cut to size at
https://www.acfgreenhouses.com... . I have purchased some in the past from them, and it held up fine for me.
If you are are relying on the Sun for heat only, adding insulation to the greenhouse is only going to slow the heat loss during the night. Eventually the greenhouse is going to reach within a few degrees of the outside temperature. A better insulated greenhouse is just going to release the heat from the day a little more slowly.
