Firstly, I should mention I play on Empire and know nothing of republic. Purple crystals are not hard to obtain. I didn't read the entire thread, so I don't know if this is mentioned. Though I saw someone say stuff like only marauder and jugg's can get a purple crystal, which is completely wrong. I know plenty of Sith Sorcerer's with a purple pvp lightsaber. Outside of that, do Battle of Ilum on hard mode. The final boss drops offhand weapons. Ninja (since the only thing that matters to you is your purple crystal) a sith warrior offhand columi token. Go to the vendor and use that token to buy the "Fall of Atris" which is the marauder offhand weapon. You can very easily pay 30k to pull the crystal out of the lightsaber and put it where ever you please. I have to say again that I can't speak for republic and whether or not this works for them.