I find the eldest stuff mainly at cheese specialty shops and cheese factories around Wisconsin, often the mom and pop style places, but also the ones at factories:
Wisconsin Cheese Mart has some 22 year old for sale online. Along with 15 year old.
Or Cedar Valley Cheese Store. They don’t seem to sell older than 10 year old online, but if you go to the store, they’ve got some 13 year old, 19 year old, and 20 year old specimens for sale. I even once found some elder swiss, an unbelievable 5 years old. Holes FULL of golden liquid which was the essence of swiss flavor!
Or Schultzs cheese in Beaver dam, they have cheddars 12 and over in the store, tho they don’t seem to list them on the website.
Or Gibbsville Cheese, my go to place since the 1960’s. My aunt worked there for decades. Their special reserve is available online, it’s a white cheddar about 4-5 years old, good for everyday use. I get it in 5 pound blocks, but re-wrap it in aluminum foil after opening it, as that really retards spoilage. Once in a while they make 5 year old cheddar caraway, which is DIVINE!
Lots of the factory stores sell items there that they don’t offer online.