One thing you could do is build a pony wall at the other end, creating an alcove. Otherwise I think you would need a corner tub. What material are you thinking of? You could tile the walls to the floor and have a free standing tub. If you plan to have the tub against the walls you will need a tile flange on your tub. You could check the different manufacture web sites. Like Kohler, Toto, etc.

Is this tub going to have a shower too?

I haven't seen a 2 wall tub available for a reasonable price. Plus, you will still need that awkward two sided shower rod and 2 curtains. If you build the 3rd wall, you can use a standard alcove tub for a tub/shower installation. Those are fairly cheap, even for good quality. You can also just use a standard shower rod and curtain then. Any time you are able to use "standard" off the shelf products, you can save tons of money over custom or specialty products.

Is this a shower/tub combo, or just a tub? It makes a big difference if you will be putting in a shower.