Buds need phosphorus and potasium for big buds, yes. Fox Farms is pretty popular, and I'm sure it works well. I've only heard one problem that they have a bunch of additives they make you buy.

Otherwise, I just use a good organic 20-20-20 for veg, and then move to a bloomer. Advanced Nutrients has some pretty good stuff, too.

Give nutrients at 1/2 suggested to start with. You have to ease your plants onto it.