This may not still be the best way, but back in Heavesnward, probably the easiest way of farming out the crystal sand was to accept any of the battle levequests in Ishgard that are located in Dravanian Hinterlands. Start the levequest and fly around the area looking for a treasure chest to open, this will have an amber encased vilekin. Abandon the leve without finishing the combat portion, then start it back up again and look for another chest.

You're still limited at that point on how many leves you have banked, but 100 leves should get you about 40-60 crystal sand.