Tons of places have them. Bass Pro Shops, Cabelas, Wal Mart, Dicks Sporting Goods, Academy Sports, West Marine, Boaters World, etc...
Any line has the capability of scuffing the gel coat, but most of them won't even come close to doing any type of damage at all. I carry three lines in my boat. One that I purchased from Academy (nylon 3 strand), one from Boaters World (double braid nylon), and one from Bass Pro Shops (double braid nylon). The Boaters World one is my favorite to use, as the line is very easy to work with and never takes a set. I could tie it in a very strong knot, and still be able to get the knot undone fairly quickly when I want to. It was also the most expensive of the three I carry. The BPS one is a close second, and the 3 strand from Academy is simply an emergency tow line and seldom every leaves my storage box.
All the best,
Glenn