Moss agate is a relatively rare drop from higher level mobs and mining points. 40-50 silver tends to be the low end that you can pick it up for off the AH. On the plus side, at your level you don't need it for much. You use it to make standard scopes (which are difficult-to-impossible to sell) and the spy-glass (another one that just doesn't sell much).
My advice is to ignore recipes that require moss agate. Stick to explosives and anything that can make you money (deadly blunderbuss, mechanical squirrel box, and even flying tiger goggles if you're the only one on the market).
Good luck leveling engineering.