first of all, i assume you know that the ship supplies towels to take ashore......
as for souvenir beach towels....i'm guessing they're available on the ship.....i bought one as a souvenir from the little gift booklet you get with your cruise documents....it was about the price of the WDW beach towels at downtown disney....not cheap but for the life of me i can't remember how much it was.....but it's a nice souvenir...