Are you the one that did the aquamarine accent tile in the large format shower? If so, good job. I am just about to tile my shower in 8x20 Daltile Identity in matte white. I had looked at the Modern Dimensions also, very nice size. What kind of sizes are you looking for? Allfloors dot com had an okay selection

It's been 12 years, but I bought slate for flooring at American Slate.. At that time, there was a store in Los Altos, now I see that they are in San Pablo. Looks like they still carry a lovely selection. Here is a link that might be useful: American SLate

I live a few hours south of you so won't be of any help with local places but I know the business that I use can order just about anything as long as I know who the company who makes it is. If they need to do a special order then the shipping could be a bit extra but if it is what you wants then who cares? Try a locally owned reputable business. :)

According to the designer’s Houzz page showing this project, the floor tile is ”12-by-12-inch honed marble tiles in white and gray. Doing it on a diagonal like that also enlarges the space visually.” flooring: honed checkerboard marble in Snow White and Tundra Grey, 12 by 12 inches, Altura Stone & Tile https://www.houzz.com/magazine/kitchen-of-the-week-classic-style-in-wood-wallpaper-and-marble-stsetivw-vs~168126510?vm=thumbs