When it comes to groceries, how do I find manufacturers coupons?

I do everything I can to stay within my means. I get ahead by planning my next grocery list soon after I've gone shopping, as well with planning meals. I take notes of weekly specials. I get specific items from my main 2 stores depending on the pricing (example: produce is cheaper at Store A. While bread, milk, meat is cheaper at Store B) I stick with store brand as often as I can. Lately I've been taking advantage of apps of stores I frequent by digitally clipping their in store coupons. But I'm hoping to find any advice as to couponing beyond the in store coupons. How do I get ahold of manufacturer coupons?