http://wowwiki.wikia.com/Crests - Go to this page and save each crest that you want. The crests will be saved in png format. Open your preferred graphic editing program and alter their size to blizzard's requirements (if needed).
If you must have them in tga format like Fizzlemizz said you can use Gimp or your preferred graphic editing program to do it.
If you just want to be able to use the images within an addon then use the BLPtoPNG converter to create a blp version. All this requires you to do after you've altered the png is to drag & drop the png file onto the BLPtoPNG converter .exe and it will create a blp copy.