It depends if you are building something technologically hard or the usual CRUD API + frontend, which is 99% of startups.
1. Hard tech:
- Go to tech events in major cities (or local if you can't afford that), look out for good speakers who talk about something tricky / obscure
2. Not super hard tech:
- Go to startupper events and look for the 2-3 developers among the sea of business people
In general when hiring software engineers:
Try to avoid professional employees like the plague, especially the coasters who did FANG work or anyone involved in some form of activism. You need people who gets stuff done, not talk politics (whether in the company or in the real world).
If you are going for employees, look for the ones who worked in early startups: no matter how small the startup, they must have solved the problems you'll face and hopefully they haven't been brainwashed with useless corporate crap.
When I hired for my startup employee n1 was incredibly talented - and he definitely changed the course of the startup, so pick carefully.
He came from my network but it fit the startupper profile who already launched a few online businesses before.