So I found this. This seems to be the lowest priced way to get into an AR9 if you want to reuse your own lower:
https://karrisguns.com/kg-Defender-9...n-endomag.html
Reviews of them have been hit and miss and most of the misses have been from long long ago around 2012. One thing in this price is a complete endomag (10 round) if you get it shipped to Colo. The endomag costs $40+ if you buy it direct from the manufacturer.

But then I watched Larry Potterfield put an upper together online last night and putting on the carry handle sight seemed to take the longest. I'm not planning on one of those.