I’ll second what Yumbo said–you should probably buy from an Asian grocery, on or off line. There are two main types of bean sauce, whole and ground. You apparently like the former and that’s what you should get if you buy a product labeled just "bean sauce". Koon Chun and Sze Chuan are good brands. Important note: These products are not the same as "black bean sauce" as noted on restaurant menus. They are the main INGREDIENT of that sauce (or can be–see below), but you have some cooking to do as yumbo’s recipe indicates. You shouldn’t attempt to use them straight out of the jar (or can).
You can also buy bean sauce with added ingredients like chilis, garlic, sesame oil and/or sugar. These should be labeled something like "Soy Bean Paste with Chile" (Lan Chi brand) or "Szechuan Hot Bean Sauce" (Sze Chuan brand) or "sweet bean sauce" etc. Even though nearly all these products are imported from China, Hong Kong or Taiwan, the labels should tell you what’s in them. Again, they are an ingredient, not something you use as a completed sauce.
Finally, you can also buy simple fermented black beans or fermented beans with various additives (e.g. ginger), which the above recipe apparently calls for. Brands included Mee Chun, Koon Chun and Yang Jiang.
Which of these various products you use will depend on preference. I’m afraid you’ll have to try a few "black bean sauce" recipes to figure out what your preference is.
I love Asian cooking but most of the fun has to do with experimenting with the condiments–a little more bean sauce, a touch or chili paste, more or less garlic and ginger, maybe a little Hoisin for sweetness etc etc. So I recommend YOU experiment, perhaps starting with yumbo’s recipe, until you get it the way you want it. If I were doing it I’d immediately substitute Shaoxing wine (NOT Shaoxing "cooking" wine) for the sake (this is Chinese rice wine rather than the Japanese sake), I might try it using plain whole "bean sauce" rather than fermented beans (it’s a little easier to handle and to get), and maybe I’d use sweet rice flour ("mochiko") or even tapioca flour rather than potato starch.