Apart from those given in quests, lvl 51-66 sprites are extremely rare to farm even with charms and gems, and they take a long time to farm. I've been farming for a sprite for more than 15 hours with a total of +150% drop rate and not a single drop. I don't really fathom the reason for having such low drop rates. Higher sprite drop rates would also encourage players to craft and collect their own stuff instead of buying from other players. A merchant that sells these rare sprites or quests that allow players to choose what sprites they want are great as well.