I have read through all of the pages of this, so I apologize in advance if this has been mentioned....
I found a vendor on the Imperial Station selling colour crystals for credits. The Magenta crystals and Purple crystals among them. If memory serves a level 50 Purple crystal with +31 to Critical (I think - I am only level 43 so can't buy it yet) was being offered for 250,000 credits.
If I am wrong.....sorry