I get most of mine from buying them on the forums, honestly. xD; It's the fastest way. A lot of people are selling for 530 treasure per gem right now, and a lot of people are advertising buying for 510. Occasionally there'll be a thread buying/selling somewhere between the two rates.
The gem rate fluctuates since it's based on supply and demand. It goes without saying, but it's advantageous to buy as low as you can.