Any reputable cyclery shop will assorted sized knobbies fro the application at hand. I use to ride with the fatest tire my 20" Schwinn would hold back in the early 70's. Back then $4.50 was almost a full days' wage for me as a kid.
In todays market of inflation, maybe a good chunk of cash.
Here is a start point: BicycleTires.com - bike tires, tubes, pumps and accessories
Good luck on the hunt