Is this an Eichler cabin ;-)
In answer to your question, I would guess most manufacturers of bathroom fixtures have a selection of standard and non-standard tubs. For instance, American Standard has a Huron tub that is 4' (rather than the standard 5'). Your best bet is check out the websites of your favorite manufacturers or post to some of the plumbing forums.
Good luck.
Jake
there is a tub on craigslist that you might like...
http://sfbay.craigslist.org/eby/mat/549101448.html
I had been looking at the Greek tub by Kohler. 4 ft long.
There is a store in Fremont that has one of the largest selections of tubs I've had the pleasure of laying in. Email me if you want the name.
J
Here is a possible solution to the tub dilemma. We have the world's smallest tub (or at least I thought we did!). There is a type of tub that hs a door on it and you can sit down in it. You may have seen ads for these and they are made for the senior market usually. My friends's parents had one installed in their little masterbath and it is really handy and effective. I had been doubtful when I looked at the as in the magazines, but my friend showed it to me after her parents had it installed. You can get different sizes, but it really fits in a small space. If we ever add on a masterbath, we might use that since we have so little space to ad on anything. It is quite amazing. I think it is a bit pricey, but
it is really amazing.