To obtain forgeries, you must speak to the owner of The Black Cat in Gran Soren. Standard items will not unlock the achievement, but there is an easy way to get it without paying any gold.
Find a ladder outside of The Black Cat. Use it to climb onto a rooftop, then use the rooftops to make your way atop The Black Cat. There is a room with an open doorway and a chest inside. Open the chest and take the Wakestone Shard. This is a forged shard and will unlock the achievement as soon as you pick it up.
If the shard is not in the chest, save your game, then reload your save until it appears.