As long as you can meet the cost, you may activate the ability
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)
Each event has four stages: beginner, intermediate, advanced, and expert
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
Red and black have the most consistent ones, in my opinion
My gratitude goes out to Phil Li (@ThePheylop) who helped me find a piece I loved and negotiate a price I could afford