Have you actually seen a full-size Buddleia hedge example? I see lots of individual plants around Portland, but not a hedge, the plants are rather sprawly, not compact like many hedge plants.

I winter sowed seeds of B. 'Black Knight' I harvested from my own plant and got like a thousand percent germination back about 8 years ago. I planted them in several garden beds here and there on my little green acre in slightly acidic sandy loam in Southern New England. Cost: zero. All continue to thrive. While the plants aren't sterile (obviously), I've never had any volunteers sprout. I mulch with pine bark chips over recycled corrugated cardboard but am still somewhat surprised never to have found any seedlings given their enthusiastic performance when winter sown.

Maybe check Santa Rosa Gardens for tips about sterile cultivars.