You can buy beach towels in stores specialized in beach items, such as department stores, sports stores or souvenir stores. You can also find beach towels at local markets or at beach stalls during high season.
In Nerja there are many Chinese shops that sell beaches, parasols and beach chairs.
Asking the hotel for towels: As for borrowing towels at the hotel, many hotels offer beach towel lending services to their guests. You can check with the hotel reception to check if they have this service available.
I lend Airbnb towels: If you have an Airbnb. You can ask for towels. Some lend towels.