I’m really not a fan of any diet that requires special food, but then again, I realize that different things work for different people. That said, right now Amazon is offering big savings on select Slim-Fast products; most are already marked down, then take 15% off with coupon code SLIM2OII and save another 15% and get free shipping when you buy via Subscribe & Save. Some of the ready-to-drink shakes come out to under $.60/apiece!
Alternatively, you can just eat directly out of the bag of spinach whenever you’re hungry. That requires checking your teeth pretty often, though.