Try the Kohler website. They have a large selection including tubs with heated backs if that is a concern.

There is nothing better than porcelain over cast iron for holding heat.

If your bathroom is small and narrow I would not consider putting a bath in it but rather use that space for a double shower unit. they are practical and look great plus you can use a portable tub in them for bathing young children. if you insist on a tub you need to consider one with full depth sides or a built in rather than a footed bath because if you dont have good access right around it the underneath will just become a dust collecter.
As far as what material to use you simply cannot beat porcelain but they are extremely heavy so if your bathroom is upstairs or in an awkward spot it would be much easier to fit a freeformed tub. At the end of the day the choice is yours so pick what YOU really like as your going to have it for a long time.

We went with a Victoria and Albert tub. Waiting for the install but the material they use is supposed to retain heat well.

Victoria & Albert tubs are fantastic. The material is durable, retains heat well, is easy to clean and contractors won't complain when you ask them to move it upstairs! I have one in my master and use it once a week. Cast iron is HEAVY but holds heat the best, acrylic tub bottoms flex and don't retain heat well, V&A might just be the best of both worlds.