Towel, or get a bidet with an air drying option (but few models have effective dryers, so shop carefully).

I have yet to find a comparison of embedded energy costs between bidet dryers and a supply of cotton towels, but I wager towels will win out when taking into account bidets breaking down. As far as I can tell, they're very repair-hostile designs, and would love to find a manufacturer that stocks all the parts for their models for a long time.