These files will work in Cockatrice and probably other programs too.
This collection lives at https://github.com/mtgtrice/mtg-decks. Contributions welcome!
The most up-to-date download is available here.
.codfiles are from this reddit thread, retrieved 2016-12-09.decfiles are from the Magic: The Gathering Wiki
The Intro Packs page for any given block on the wiki should get you .dec files
for any theme deck in the block.
Example: http://mtg.gamepedia.com/Khans_of_Tarkir/Intro_packs
Lots of decks for download at the tappedout deck builder!
Another project by Asthner, who made the original thread this project was based on.