beach tanning towels?

I don't think the item is in buy mode. When sims do the ''relax'' option at a beach, a towel (and maybe a parasol) will spawn in the spot you selected and lay on it. I've seen sims catch a tan either sleeping or reading a book, enjoy.

I believe it came with Seasons.

If I remember correctly, there is a certain strip of area that needs to be clicked within for the option to appear.

Sunbathing came with Island Paradise. It's here in the official trailer... :)
https://youtu.be/4BTIGeKpepY

Okay, going back through EP content:

Sunbathing came with Seasons. You can lie in a lounge chair or the pool float (or just stand outside). There is also a tanning booth.
Sunbathing on a towel came with Island Paradise.

I remember my Sims spontaneously combusting when I didn't want them to (trying to score the badges) after Seasons came out. :D
IP added the towel option in the mix.

If you go to a beach (so far I've only had success at community lots designated as beaches) and click the sand, there's an option to "relax here". A towel automatically appears, you don't need to buy one :) I also recommend bringing along a radio or boombox in your inventory and then taking it out and placing it next to the towel. it's nice!

http://farm8.staticflickr.com/7459/9166149501_96d46b8f35_c.jpg

I think it came with Island Paradise - so if you don't have that EP you won't get the option.

It's not in buy mode, your sim automatically lays one on the ground when you click the option to suntan

You can buy them in outdoor activities. Or just do a search for towel in outdoor activities. You can pick from several patterns.

Hey there it's awesome you're wanting to help out but this thread has been inactive for quite a while so I'm going to close it off. If you have any questions about necroposting you can read this thread here : Necroposting