Well, 9mm carbines are definitely available, a few are not out of my budget, and ammo for target practice is easy to get and not too pricey... (Our Rural King stores were running a special on 9mm target ammo recently, which, plus links like the below, were some of the things that got me thinking 9mm.)
https://gundigest.com/rifles/affordable-9mm-carbine-options
The HighPoint referenced is a good option. It is butt ugly but low cost and functions well.
The Kel-Tech and Ruger are also fine. Also, Beretta makes one as well but I suspect it is pricey.
Build an AR-9. Cheaper than you think.