As a lefty who started his first build in 2015, I know how you feel. After years of being at this game, I'd recommend NOT trying to find a barreled action in LH.
Your best options are as follows:
- Buy a factory LH gun and work with that
- Buy a Canadian made, Ultimatum Deadline barreled action
- Buy an action and barrel separately and then build from there
I kinda mixed and matched these for my first build, bought a Tikka rifle, stripped it down to the action, bolt and trigger then built it up from there and I love it. The reason I'd advise against buying a barreled, custom action is that you have limited choice on both of those components. Don't get me wrong, I like both the action and barrel from UP, but it wouldn't my first choice if I were to do it again.
What I'd recommend you do is to contact Gary at Bighorn sales, Ken at Bullseye London or Tom at Go Big Tactical, look into a mid tier custom action like a Defiance Tenacity, Bighorn Origin, Kelbly's Atlas Tactical or ARC Nucleus and then build from there

If you have any specific questions, feel free to PM too!