This is probably better suited to the Light House, but...
Your options are either the Homebrew Channel or a modchip. In order to do the Homebrew Channel, you need a copy of Twilight Princess and to get the Twilight Hack (a good place to get it is
here), as well as an SD Card and a way for your computer to read SD Cards (either a built-in or separate SD Card reader).
A modchip would give you more options, but it severely voids your warranty, would be hard to install, etc.
You should probably own Twilight Princess anyways, but if you're looking for it cheap just for this, I'd suggest renting it. The cheapest way other than that is probably somewhere in eBay or Amazon where there may be a cheap used copy (I'd highly recommend against GameStop, their prices are pretty crappy).