Do a web search on some manufacturers of bath products (Kohler, American Standard, etc) and you'll quickly have more choices than you want. Just did a quick search on Kohler's site and found a 54" bath for 3-wall alcove installation (for example).
We've been looking into replacing the bathtub in our Eichler.
In an alcove installation, the tub has a skirt which is usually directly under the tile. So if you're measuring tile-to-tile (or otherwise the interior dimensions of your tub surround), you've actually got at least another inch to work with.
In our case, even this left us at 59 1/2, but it turns out you can even tuck the skirt under the cementboard too.
So - long story short - we're shopping for 60" tubs.