Remember, the stiffer (harder, more dense usually) the belly, the thinner the total stack. Thus, so you don't run out of belly to work with,that means make the core lam thinner.
Loon mentioned black locust, which works great, but is heavier than you strictly need. My best luck was with tough, but lighter woods(remember, the middle lam is under very little compression strain, but some shear strain). I like maple, cherry, bamboo flooring, light end white and red oak, maple for sure, and I once bought a thin red elm board off Ebay as a 'throw in" on kind of a package deal of thin pieces. That was great stuff, though a little hard to mill thin and smoothly.
