Sacred Source is (or was) the primary seller of Mesopotamian themed idols for a long time. I got my water-pouring goddesses (see banner image) and votive figurines from them. However, a quick search shows me they no longer offer either of these pieces.
Another seller I've gotten pieces from is the DownUnderPharaoh, who has a shop on both eBay and Etsy. All of DUP's merchandise is hand-made and of excellent quality and condition.
The woman who runs Cimentarius is also very cool. She hand makes all of her orders and sometimes take requests, if she has the material / skill to complete them.
Finally, every now and again eBay and Amazon will have a piece available. I got this piece, which is actually Shamash and not Ashur. They also sell lamassu (#1 and #2). Good search terms are the names of deities followed by "statue" or the names of the cultures: Sumerian, Akkadian, Assyrian, Babylonian.