A common brand:
https://www.bobsredmill.com/catalogsearch/result/?q=grits
I personally prefer the white corn Southern Style grits but either the yellow or white works.
My favorite is stone-ground grits from one of the southern states, but you usually have to order those from small, boutique type mills. The ones in the link are all long cooking grits versus the instant that Craig mentioned or the quick grits, which he didn't. I can't stand the instant, they are more like cream of wheat. I don't really like the quick grits either, which are usually cooked 5-10 minutes. The directions on most long-cooking grits usually call for 25-35 minutes of cooking, but I cook mine much longer than that, more toward an hour, adding additional liquid as needed. I feel the taste and texture both improve. I also use a mixture of liquids, stock of some kind (usually either chicken or shrimp), cream and water, plus butter and lots of salt and pepper.
You have to aggressively season grits or they are just bland, just like polenta. I think that's the reason a lot of people say they don't like them. They've never had them cooked correctly. When I do order grits in a restaurant, and I ask how they are cooked prior, I ALWAYS end up adding salt, pepper and butter because they are never seasoned enough. So, I understand why people don't like them.
Grits can be soft and slightly runny, or firm enough to chill, cut and fry until golden brown and yummy. Just like with polenta and risotto, the amount of liquid you add dictates how "tight" the end product is.
Grits are more popular in the southern portion of the U.S. Except for maybe in the large cities in the south, you won't find a restaurant that doesn't have grits on the menu at least for breakfast.