or any normal supplement store, it doesnt have to be mass attack though, any decent weight gainer... do a search and see what you can get for a good price
I see you said you were having trouble gaining weight!
If you get your diet in order using 500 calories more than you need in a day will put weight in...
Easy enough to say, but if you buy some weight gain powder (I'd advise boditronics mass attack) you can get the calories easier. remember to eat or have a shake every 3 hours without fail!
If you write down everything you eat for a week, then go through and see where you could improve, the next weeks diet will be 10 fold better.... Little tips like that will help you put on them pounds!