I am a software engineer with experience in multiple startups, both as a salaried employee and as a co-founder.
In the software/startups that I develop on my own, I am very lacking in marketing and lead hunting. My projects usually have a good growth potential, but I fail in the non-technical parts.
Of course, I have tried to get another co-founder to work on the non-technical side before, but I don't know how to find a good non-technical co-founder.
What would you suggest?
This implies equity of some sort in your company. And I doubt you are really prepared to deal with all that this implies unless you know the "co-founder" really, really well --- and even then it can be dicey.
Instead, look and advertise for people seeking "business opportunities". Start by offering a $0 startup to others looking to start their own business built around a product they will source from you.
These other individuals/"partners" will be responsible for setting up their own company, doing their own marketing and billing and first line support.
Establish a clean line of separation by charging these partners a fixed "wholesale" rate and providing them with an "opportunity" to make money by re-selling/marketing your product. They will have total control over their own destiny as you will have over yours. The more they sell, the more they make --- and the more you will make.
Once you have "successful" partners, you can leverage that "success" to actually start charging others a significant upfront fee to become part of this successful enterprise.
The key to making money is offering others the opportunity to do the same.