customizing your own mix is pretty convient and you know its quality. But will run you few bucks more per lb, depending on what type/quality of protein you get. IF looking for a manufacturer's protein, Optimum Nutrition's 100% whey probably the most popular choice (can be had for low 40's for 10lbs, low 20's for 5lbs) I use that, along with EAS simply whey (similar prices). Others will chime in with pro-rated (fantastic deal for decent quality), isopure (all isolate but bit pricey), and others from sportsone, universal, next nutrion's designer whey.... but i would recommend you getting the 2 earlier mentioned powders.
As mentioned before, all gainers should be HOMEMADE. Source of fat (flax, pb), 'quality" carbs (oatmeal, flax seed, some fruit, milk), along with you choice of protein.
Sage