That is one big frickin kitchen.
I'd guess Lowes or HD would be very happy to work with you on an order
of that size. They do sell prefab cabinets off the shelf and maybe
could order ones to match.
Two tangential things -
1) 20x30 is a monstrous kitchen, larger than most people would want.
Half that is a big kitchen, I'd bet 95% of all homes have kitchens
smaller than 20x15.
2) If want to construct the kitchen in such a way as to someday
potentially attract buyers who want a big kitchen, those would be
high-end buyers, who would probably expect high-end cabinets, not the
ones on the shelf at Lowes.
That's a very good point about high-end stuff... but the reason we're
doing this is because we love the house and never plan to sell it...
even though it's only 3 bed 2 bath.
20x30 includes a "laundry" nook, and we're going to have to build
cabinets around the freezer, two external doors and a double window as
well, it really doesn't feel like that big of a room when you're in it.