What is this called and where can I buy a replacement?
This is the throat block for direct drive extrusion, Anet8 is a cheap clone of Prusa printers, so it's easy to find parts for Anet printers. 

This is one extruder kit that may help your needs, 1 Unidades Impresora 3D makerbot MK8 Extrusora De extrusión de Aluminio Bloque de DIY Kit para Reprap i3. This is a link for Aliexpress - of course there are other dealers.