Unfortunately mithril and shards are pretty rare. Keep collecting from your tablet in the hope of getting shards. Mithril has been reported from mining (especially dark cave formations), sparkling mines and sir piggles. Don't focus too much on getting mithril as it can reduce your enjoyment. Keep playing the game and you'll be super excited when you actually get it.
If you've got the love shack you can
As I can't see into your game I'll ask a number of questions. The questions are in the order you need to follow to get it.
1. Have you expanded to Thurston (guy in his boxers)?
2. Have you dressed Thurston?
3. After dressing him have you seen a skunkupine to get defeating peril quest?
4. Have you finished defeating peril quest?
5. Have you crafted the ballista and collected 60 stout guards?
6. Have you placed the building in your kingdom?
7. Have you finished building it?
8. Have you stored it? Is it in your inventory?
9. Did you sell it?