This proves to be harder to answer than I would have thought. Elly still shows up in the freestuff but the file can no longer be downloaded. Looking through my collection, I can only find one that really fulfills what you want, which is Dina by Lisbeth. But checking in the RMP, it seems to have been removed. Other than that, Arduino's Tizian does have some tan lines.

There must be others. After all, there are so many textures out there.

Incidentally, for purposes of making your own tanlines (which I know is not the OP's wish), it seems to me that one could take advantage of bikini "second skins" to identify the areas of skin to lighten. This way it could be a very easy process. Add the second skin as a layer in Photoshop over the skin texture to be modified. Select the bikini colour area. Feather slightly. Invert. Now move to the actual skin texture, darken it. Remove the bikini layer and save. I think I'll try this myself.