There are two types of pieces: Lord pieces and Quality pieces.
Lord pieces are used to unlock specific Lords.
Quality pieces are used to educate Lords of the same quality.

How do I get pieces?

With the release of the 8.0.0 version both types of pieces will be given to you through normal events and tasks. Meaning: The more active you are participating the more pieces you will be able to obtain. It is planned that it is possible to buy pieces in the alliance shop or be found at alliance tournaments.
Additionally, during big events (Halloween, etc.) there can be a higher drop rate of these pieces.

What happens to excess pieces?

If you already unlocked a Lord and still get the corresponding lord pieces, they will automatically be transmogrified to quality pieces depending on the quality of that Lord.
Example: Robin Hood is of the Common quality and you already have unlocked him. You find additional Robin-Hood pieces. The Robin-Hood pieces are automatically transmogrified to Common-Quality pieces.