Rowan Big Wool or Lion Brand Big might be good substitutes. They are both plied yarns (several strands twisted together) unlike Waterspun Weekend, which was a felted singles (one thick strand), so the effect will not be quite the same, but if the gauge matches you should be okay. I made a pullover with Weekend and was not thrilled with it -- it pilled like crazy and because of its singles structure, had a tendency to pull apart as it was being knit. You might actually get better results with a substitute.
For other suggestions, go to yarndex.com and click on Super Bulky.