Discounttiredirect.com is good website for finding cheap mud tires for your car. They also have free shipping.
The function of a mud tire in general is to help the vehicle navigate its way out of mud or prevent the vehicle from getting stuck in mud. If one purchases cheap mud tires, they may not be as effective.
I would go with well know tire companies such as tire kingdom though craigslist will have cheaper prices.
Mud and Snow
One can purchase cheap mud tires in a variety of different stores. Check Craigslist and nearby local auto dealers. Amazon may also have cheap mud tires.
Companies that offer cheap rim and tire packages are Auto Zone and Tire Rack. These two companies beat all competitors in providing cheap rim and tire packages.
You can buy mud tires from a local tire dealers and from many online tire stores. Good thing to remember when buying mud tires is that cheapest isn't always the best.
Mud & Snow.
Super Swamper TSL or a Bogger
Discount Tire does offer cheap tire and wheel packages. The only caveat however is that you would have to purchase your tire and wheel package separately. There are no combination packages on the official website.
Any tire that sticks out past any fender (looking at tire from the front to back) that has potential to throw anything backwards towards other vehicles, shall have mud flaps. If there are no fender, mud flap should be used.
It is possible to get cheap tire and rim packages from stores like eBay and Tire Rack. Amazon may also have a discount on these tire and rim packages once in a while.
Walmart, Sears, American tire depot, Discount tire, Online tires, Conrad, Dunn tire, Tire warehouse, Simple tire, Just tires are the stores that sell tires for cheap prices.