This is a bit of a 2 part thing, you need the raw crystal but also someone with a recipe to turn it into a lightsaber crystal -
1. Go to Tatooine and get the water buff. The one from clicking a water tower panel
2. Go to Hoth and climb the highest mountain. The closest base for Horde....erm I mean Empire...is Lesh outpost, so will be equiv for Republic. you can see the mountain (well its the highest one, what more do you want?). The water buff will be exhumed and an item 'Shockfrozen Water' will move into your inventory.
3. Needs a group of lv50s. Tank, healer, dps set up. there is a cave on Ilum, not sure where but can easily be looked up. Go in, click an item, beat a lv 50 boss mob and a crystal mound is spawned. loot this for the crystal. Only 1 person at a time i'm afraid.
Now you just need to either find someone with the desired crystal recipe and probably pay have to pay about 30 billion ISK for the craft, or if an Artifice, you get the recipes (there are 3 diff ones) from higher level world bosses