Nope, they do not have xtal sales as far as I know. You can get then from quests, buy them with real money from the Oracle, or get them free from the devs every now and then.
For future reference, you receive 20% of the cost to get to that level. That being the cost to build one to its current state, not what you paid for it.
It's from a spell that costs mithril in the alchemist. It's temporary by the way, not permanent. To unlock the alchemist it requires a level II castle. To get mithril you have to war (or buy it during one of the various mithril sale).