You can use pre-cooked beets for convenience. But know that cooking beets is known to diminish nutritional quality so your resulting juice will be slightly less nutritious. For instance, LoveBeets brand steams theirs for 40 minutes. If you do choose to use pre-cooked, you can find them in packages in the produce area of grocery stores. Be sure to use their plain flavor, however, don't buy a flavored (vinegar) version by mistake.