The Brazilian tulipwood and Indiana tulip poplar are very different woods. There are plenty of commercial sources for tulip poplar. It is the softest of the hardwoods and can be nearly white or streaked with green, brown or dark purple. With the brown/purple coloration is is usually called mineral poplar. Antique poplar is commonly called yellow poplar and is usually a greenish yellow color.
As for sources, it is available at any of a dozen or so sawmills within 20 miles of where I live in Southern Indiana. Most of the trim in my house is made of poplar, I operated a casket making business for several years, we made them out of poplar. It isn't hard to find but is mostly too common to be offered by the exotic wood vendors.