I think you can find boulder in Colorado...
Seriously, there's a place in Waldorf at the intersection of route 5 south and Billingsley road called Longwood Gardens Nursery that has them. 301 645 5301
Yeah I got mine at Wentworth Nursery on 235N and Oakville road. You know you're close when you see their advertised pallets of stone on the grass on your right just before the right turn onto Oakville Road. Can't miss it. They also deliver!