The Darkmoon Faire comes to town once a month, and if you want to get one of those amazingly good darkmoon trinkets, you're going to need a fairly large pile of gold. There are a couple of ways of going about it with varying levels of cost and effort.
Interestingly, our own Fox Van Allen became a "Darkmoon Mogul" last expansion when he made his first million gold. At the time, you could simply make as many cards as you were willing to mill the ink for, and people willing to make those cards could profit in spades. There's still money to be made in this, however don't walk in with your eyes closed -- the system has changed, and there's more risk now than ever.
Getting one the hard way
Getting a deck for yourself can be expensive and simple, or cheap and complicated. You basically have two choices: hustle for cards to make your own deck, or buy a finished deck or trinket.