I've found source code for a Z8000 BASIC interpreter and monitor, in an online book by Rolf Dieter Klein. I did a quick check of the OCR. My Chrome browser can translate the German to English, not perfectly but fairly readable.
The OCR'd listing is very poor, so that will be a lot of work to restore to usable source code.
Jurjen Kranenborg reports he is developing a monitor and BASIC interpreter based on Klein's code. https://www.kranenborg.org/z8000/software/index.htm but no sources are visible yet. I have had a go myself, but have no Z8000 assembler yet.
Lacking an assembler, and needing a reference to check an assembler output, I converted the book's listing into something that can be assembled as a collection of labels, hex words and ASCII characters. The original assembler statements are made comments. This involved a lot of editing and typing the hex data in by hand!