You might want to consider having the enamel resurfaced on your existing tub.
- There are many businesses providing this service.
- Lead-free enamel paint is available; in fact, lead paint is not allowed to be produced in the U.S. any longer.
If enamel resurfacing is not an option, I would recommend a cast-iron tub with a baked-enamel finish. This is the most durable type of tub available; the finish is lead-free; and most metal products include a significant percentage of recycled content.
For more information:
Read "What's less toxic: refinishing an old cast-iron tub or getting a fiberglass tub?" a Q&A answered by Molly McCabe.
Also, read Mick Dalrymple's Q&A "I have a bathtub that needs re-enameling. Is there a nontoxic way to do this?"