I live in the USA and Cumberland sausages don't exist. They have sausage links and patties but I don't want to make sausage casserole and have the flavor altered because I picked the wrong kind of sausage. Can anyone help me?
If you read the Wikipedia article you'll see that the main flavour is pepper, both black and white,and that the meat is coarser than in some other sausages.
As it's for a casserole where the flavours have time to come together, I suggest you get sausages without herbs in (which should be easy enough). If possible they'd also have a coarser texture but that probably isn't on the label - you might have to choose based on what you're familiar with. Then add plenty of pepper to the sauce, early in the cooking. My recollection is that the white pepper flavour comes through, but I use black pepper quite heavily so maybe I'm used to that.
The other spices (and herbs though they shouldn't dominate) vary. This recipe includes sage, which other sources (and I) say is wrong, but the nutmeg, mace and coriander all contribute to the classic flavour. While I haven't made it, the recipe I've linked should give some idea of the proportions of spice to meat,assuming your sausages are quite bland to start with.
Edit: I should probably have been clearer that I mean links. Sausage patties aren't really a thing here in the UK. I've seen them twice. Once was McDonald's, the other may have been good but I can't remember.
Cumberland Sausage is thick and made in one long ring. Coarse pork mince - either hand-cut or using a very coarse mincer. Ingredients for the spice mix: white pepper, black pepper, salt, sage, thyme, mace, nutmeg, cayenne. Use very little mace and nutmeg - but they are a key to authentic Cumberland Sausage flavour. Casing for the genuine thing has to be natural pig intestine.
This is a tough one.
To purchase Cumberland, I found:
Keswick appears to only be a walk-in market. Proper and Parkers, on the other hand take internet orders and ship.
You could try and make your own: There's this How to make a Sausage video. I can't vouch for it since I didn't watch all 20 minutes.
Or for substitutions: The description of Cumberland being peppery made me think of Bob Evan Hot Sausage, which is readily available and economical throughout the US. It is not equivalent of Cumberland, because it will have savory spices such as Sage, Rosemary and Thyme. Maybe you could use it on one of your trial runs.