Why do you need a co-founder?
But sometimes you need someone emotionally invested in it. Cofounders are much better. They don't necessarily have to have 50% share. I'd be happy to take on someone whose sole purpose is to be the customer surrogate and give them a 20% share.
My cofounder helped to keep a leash on me doing really dumb things. All good entrepreneurs are relentless, and a cofounder helps to keep it in perspective. Also a cofounder adds their network, and my minority share co-founder landed the acquisition.
If you let co-founder in you are not only sharing % of the company but also vision. Getting co-founder is like getting married, if you struggle to find someone you know and trust maybe it's better to stick with advisor for now.