American black walnut is what you see the most of, although ipe, aka "Brazillian walnut" has been used by some. It's VERY VERY hard though, especially compared to American black walnut's relative softness, so it's more difficult to work with and usually has a higher labor charge as a consequence. American black walnut is a premium species, so commands a pretty high price by itself. Also, you will need to decide if this is a worktop, or a pretty top. End grain with an oil finish is recommended if you want to cut on the surface, and face grain or veneer with a more durable cabinet grade finish is what is recommended if you want something pretty, but mostly just to place objects on it. The really pretty grain pattern walnut pieces that you see are typically veneered, and possibly bookmatched if the piece is a showpiece.

I have yet to visit this showroom. But at this point we plan to go with this company.
I don't know what area you are in so shipping could be a deterent, but maybe there is a place in your area like it.

Here is a link that might be useful: Butcherblock

I went with this company for my Mesquite bar counter top material.
Lots of Walnut examples in their gallery shots....

Here is a link that might be useful: William Rogers Wood Tops

Ooooh, my next house! (those wood countertops are luscious!)

Thank you all! We will not be cutting on this top so I'm thinking the black walnut sounds perfect

Of all the walnuts in the world, American Black Walnut would be your least costly option. If you can afford it, specify all heartwood. That will give you a uniform dark wood surface.