A local butcher or aboutiour (spelling is not right) will have it. Just about any farmer who sells 1/4, 1/2, or whole cows from their location will have it too
If in some places you're having trouble finding grocery stores that will sell fat, check out Mexican markets. They usually have all kinds of fats and trimmings.
+1 for a butcher shop. If you get to know them they will hold back some trim for you. I usually get some really fatty shoulder for ground. Bacon ends are great but they tend to make things taste like bacon which is not a bad thing. I can order beef fat from mine by the pound but it takes 2-3 days to arrive from their wholesaler.
Pork belly is my go-to for any caribou or moose sausages or salamis since I can easily find pork belly in Alaska, whereas finding good cuts of fat on short notice can be a challenge. For burger, I use either bacon ends, beef fat, or pork belly. Nobody complains about burgers that have bacon flavor!
The fellow that said Mexican meat market is spot on. I can get any fats I want and half the time it’s free. Other stores I have to call ahead, wait, pay, and hope it’s what I wanted.
I do quite a bit of cooking on my smoker during the year. I will trim 5-8 lbs of fat off some briskets and vacuum seal them. If I'm lucky enough to shoot something I usually have 80-100 lbs of prime brisket fat that I grind in with my game.
We never grind any in. Just unnecessary. You can always add some sort of binding fat in when cooking. Just gunks up your grinder. We render our own pork lard from hogs we butcher, easy to throw a spoonful in the pan or bowl when cooking.
We used to buy pork fat from the local butcher. I didn’t mix any fat into the last batch I ground myself because it almost exclusively goes into recipes that the meat isn’t a “main attraction”.
I personally like to mix lamb in with wild game. Very fatty and doesn’t change the flavor imo. Also don’t have to worry about cooking temps as much when using pork.
Most butcher shops or grocery stores require a couple day lead time. I’ve had good luck at Carnicería’s or local Mexican markets for same-day beef fat. I use bacon lot also but remember not to go rare on the burger if you mix bacon in as the grind,
For beef I use suet (fat around kidneys). Its dry and crumbly and mixes very well. In my experience it tends to make whatever you mix it into taste like beef. Pork is more neutral flavor. The best fat pieces I've found are from the back. Fat "trim" tends to have some connective tissue and whatnot so isnt quite as clean to cube up and mix in. Both can be had from local butchers but in my area i have to call ahead so they can set aside.
I use pork back fat for most applications. It’s flavor-neutral, and lends a good texture. Maybe 20% of the time I’ll do a batch with really good beef suet that makes for really great burger - specifically hamburgers. It’s unctuous and extra beefy.
For either I shoot for about 15% fat by weight. As low as 8% and as high as 20%, but that’s all down to preference.
We just use the cheapest fattiest pork shoulder from any grocery store. I have also tried straight pork fat from a meat counter and bacon ends and bits which were to salty for me. If you know a farmer you could get really cheap or free fat when they butcher.