So, to answer your question, here's what I did.
I typed "Moss Agate" into the Search box.
I got one result back under the category "Items".
I clicked the link to view the item info page for Moss Agate.
I see that many mobs have a chance to drop it, and that is can be found in various objects like Tin and Silver Veins.
I scroll down and find the comments to be interesting as well, as far as what instances prove to be reliable in dropping Moss Agate.
I hope this helps you now, and helps you in the future when you need to find this type of information.