I haven't seen the heaters at walmart, but that doesn't mean they don't sell them, look in the outdoors / camping section. Walmart does sell the 1 lb propane bottles and they sell the 20lb tanks , the 1st one is like $40 , but then it is on an exchange basis and around $17-19, depending on the cost of propane.
I know you can get the heaters, hose, and line filters at Most bigger outdoor stores, Bass Pro, Cabella's, dicks sporting good, etc..........
I exchange my tanks at lowes....
PS: Mr. Heater makes 2 different heaters, 1 that holds only 1 bottle and does NOT have the blower, and the BIG BUDDY, that holds 2 bottles and has a blower to move the warm air a little (get the big one). I also use a small fan hanging off a "s" hook from the ceiling about 2 foot in front of the heater to move the hot air off the ceiling ov the van, works great!!