The market wards have a search system in place now. Venture to your most popular city and search the prices on the shards/crystals and you will quickly find what you are looking for.
Forewarning, they are not as expensive as they once used to be. Unless you farm them constantly you are better off saving them for your own synthesis. Of course there are the very few people who do not craft at all.
As a secondary source, though not as updated and accurate as the Market wards search, try using the Yellow Gremlin FFXIV database, found here