Myoplex is great stuff, but it can add up.  I'm in the US and my favorite alternative resource is vitacost.com.  They have a protein mix that's sweetened by stevia.  Remember, what you're looking for is a high-quality, low calorie meal replacement.  You can find lower end solutions at big box retail stores.  But I'd suggest focusing on good, high quality whey sources that can be easily mixed with water or milk.  You won't get the convenient packages of Myoplex, but you just may find what you need to get you through the challenge.  (If what you find turns out bland, try adding Nestle Quick.  It's basically just cocoa and sugar.  Sometimes just the touch to make a blah whey product a lot more palatable.) Best of luck on your challenge!