The following thread contains extensive discussion about when to purchase tickets:

Domestic airfare

Optimal timing is contingent upon a number of factors (e.g. route, season, airline). I suppose I'd start looking ~4 weeks out in your case if I was feeling super anxious, but odds are, the picture won't change a whole lot until the last several days before your target date.

And, all things being equal, I prefer third party channels (as opposed to booking direct, which often isn't especially direct, anyway), as long as the specific third party offers good customer service. Spring is a bit of an exception, though, because their pricing bundles are somewhat unique. Even then, changing to a different airline is a lot easier if you buy from a neutral source.