is there anywhere where i can be guaranteed to find a rare lightsaber colour crystal?

 

(i have silver)

Look in footlockers and plasteel cylinders. It's rare but I've found them in there before. Also try Daraala.

Apart from cheating and discussing your lightsaber with Atton, I don't think there is a guaranteed place you can find one. I believe the loot is generated mostly at random.

Well, Silver is pretty rare, but at

Malachor IV

is the best place.

I think you might be able to find some on Dantooine in the Kinrath Cave. I found an orange crystal in a footlocker on Korriban.