Not a lot of options in that size, I know. I found the Kohler Archer, but only in acrylic. Anyone know of others?
You might want to check out one made by Sunrise Specialty that's almost 36 in. by 36 in. This is from their catalog "... #891 shower pan which measures 35 1/2" length, 35 1/2" width, 6" height. Cast iron construction and anti-slip bottom for safety." Sunrise has a reputation for making good quality cast iron clawfoot tubs and I imagine that extends to their shower pans as well. Vintage Tub and Bath sells the #891 along with accoutrements. Search on www.vintagetub.com.
If you have room for a 48 in. by 36 in. one, Kohler makes the Kathryn. Search for "Kathryn shower base" on www.us.kohler.com.
Here is a link that might be useful: #891 construction specs from Sunrise site
Thanks for the link. With that bowl shape, though, the standing room shrinks quite a bit compared to the more "square" acrylic ones I've seen. I'm also considering Swanstone, but cast iron would be nice. Can't squeeze another foot out of the adjacent room, though!