Due to a new change, Resort guests with a valid magic band now get free towel rentalThey do not provide them. I believe you can rent them. I bring my own. I have seen others bring towels from their resort, although I don't know if this is actually permitted.
Due to a new change, Resort guests with a valid magic band now get free towel rental
http://www.wdwinfo.com/disney-theme-parks/blizzard-beach-water-park/