Powerstrokeshop.com - PSD and Ford Superduty Parts and Bob Utter Ford Parts - Genuine Ford Parts and Accessories would be places I'd check out.
I believe the heavy X codes were either standard or optional on F450-550 rigs,you'd have to give those guys a call to be 100% sure
Tell us some about the year,model and drivetrain of your truck.
The X codes ride very firm maybe a little too much for my taste,the X codes are primarily installed from the factory for snowplow/Heavy Truck applications so if that gives an idea of their capacityFound them at my local dealership(Pensacola) for $167.00 per spring plus the bolts and other stuff. Not much different in price than a level kit...my only concern is how much will the ride be affected by these springs???
FWIW, I really think my truck rides better than it did with the V-codes. It raised my front up 1-3/4".
Barrett