Take the complete blade assembly off the arms (easy) and go to your local HD truck or NAPA parts house. Show the fellow behind the counter what you want, and he should be able to match something in his stock.
These blade assemblies were used on trucks, skoolies, etc., so they're fairly common, but not at your local WalMart or auto parts store.
Or, as Brian suggested, call Luke (1-888-262-2434) and support one of the businesses that supports our little hobby.
FWIW & HTH. . .