Yes, there were one or two third party assemblers available. From memory, the issue was with the downstream tools - so for instance on CP/M 3.0, I think you had to use the DR one to be able to build an RSX (equivalent of a TSR under DOS. You could count the number of 8080 CP/M 3.0 machines on the fingers of one foot.

You've reminded me that I have a 380Z or 480Z in the loft - I must get it going again.