I don't know much about weight gainers, but I was reading the label on muscle milk and couldn't believe the calorie content. 2 scoops with low fat milk was 638 cals.! 4 scoops of that stuff throughout the day with 4 or 5 small meals would be enough to put the pounds on I would guess.
You could just increase your food intake if you cant find a weight gainer ? ,add some good fats to your meals to up the calories, hope that can be of some help to ya.