There may be others out there, but these are the big ones: classified ads (of which Craiglist is likely the most popular), Ebay, and your Magic social circle (this includes Facebook groups, friends, local gaming stores, and players in your local area)
They get better when they offer good keywords, too, and first strike is one of the best for a tribe that wants to be on offense
Buying a box rather than individual packs is the most common way to open a large quantity of cards at a better per-pack price
As long as you can meet the cost, you may activate the ability
Dig A Little Deeper is the Sapphire Sing Together card, letting you look at a massive seven cards at the top of your deck before choosing two to place into your hand
Faerie Vandal