Reddit mentions of Kaiser Dill Pickles Gallon 12-14 Ct Jar

Here it is on amazon Linden Sweden Twixit Seal and Pour Bag Clip, Red

You could just use the top of a plastic bottle. You can do something like this or this

It'd take something larger for pet food, maybe like a big pickle jar? I haven't used this method for anything particularly large or with a stiffer bag material