I buy frozen organic corn tortillas at Whole Foods....10 to a pack for $1.29. I lay them out on my big electric griddle/skillet or place in my toaster oven or under broiler and cook till crisp, turning halfway through (about 4 to 5 mins). You can also use regular soft corn tortillas.
Cook ground beef, and add taco seasoning (homemade or in a packet). I use homemade...very easy! After ground beef has become prepared taco meat, mix in one can refried beans. I use a can of organic refried pinto beans with roasted chili and lime (around $1.39 a can, 365 brand - their own generic brand)
Prep all ingredients that you enjoy on tacos (lettuce, onions, cheese, tomatoes).
Spread bean/meat mixture on tostada shells. Spread sour cream on. Top with lettuce, tomatoes, onions, cheese and whatever else you like on your tacos.
Recipe by Creative Kitchen at http://creativekitchenadventures.com/2012/05/05/taco-bean-tostada-the-good-old-days-curiosities-about-homeschoolers/