If you can't find any fermented black beans (or you don't want to wait for the delivery times online) then you can buy a ready-made bottle of black bean sauce. No one will ever know the difference!
If you want a more authentic taste then go for a concentrated paste-like sauce as these don't contain any extra preservatives or additives. You can use this as a like-for-like substitution in the recipe.
If you want to use a bottle of stir-fry sauce then use this as a full substitution for the sauce in the recipe (skip all the below sauce ingredients, in addition to the black beans).