Depends on what you are looking for.
In 4x8 sheets, hardwood veneer over fir ply or MDF is available at several yards (most mentioned already), as well as home centers. Birch is a commonly available veneer. "shop" birch is a utility grade of the same, usually the veneer is flawed or ugly, and they put it on a lower grade of fir plywood (voids not filled).
You can also buy 5x5 sheets of Baltic Birch plywood. It is very high quality for the price (about the same as shop birch or a bit less - I haven't bought shop birch in years though), but is usually not as flat as premium veneered plywoods. Not all vendors carry 3/4" thicknesses in this stuff as the 1/2" is the most popular. All the plies in this stuff are birch with a very hard, water resistant glue (the glue will dull steel blades fast). I think Albuquerque Hardwoods carries this but I'm not sure - I don't shop there much. Rugby Architectural Supply is where I buy it, but I'm not sure they still sell retail. Alpine Builders supply in Santa Fe carries it too, they are retail/wholesale but do not accept CC payment (at least they didn't last time I was there) so bring cash or a check.
I mostly use Baltic, it ran around $40/sheet for 1/2" last year when I bought some. Top grade veneer plywoods are probably well over $120/sheet now, but like shop birch I haven't bought any in years. Last time I bought shop birch it was $40/sheet or so, and 1/2" baltic was $20 - but the dollar exchange rate also plays with the Baltic pricing (its mostly from Russia).
EDIT:
Hunter and Groff are more building materials suppliers - studs, OSB, drywall, etc. I haven't been to Groff in years (fair warning), but that is what I recollect. I think it's down to ABQ Hardwoods and Rugby for hardwood lumber and cabinet grade plywood. I do use the Hunter yard in Edgewood now and then. Same name, same industry, different businesses. I asked about that once and was told something vague about siblings not getting along and splitting up the family biz - no idea if that was accurate or not!