Hmm. Let's see.
1) It’s very hard to convince a cofounder to join an unproven business. And that is going to waste a lot of your time looking for someone. When you could be building your business instead.
2) You likely won’t need to build anything to validate your product if you use duct-taping, wizard of oz, lean startup. And after it's validated, well, just go ahead and build the mvp using the minimal toolset.
3) You can hire someone from a site like Upwork and in many cases they will be a lot cheaper than you are and still pretty good.
4) With frameworks like rails, laravel, bootstrap and a mountain of plugins it's pretty easy to build products that can make money in your spare time (10h week).
> Billing code, dashboard, homepage, getting users, retaining users, design/ux, debugging, customer service/support. This takes months of grueling work.
If any of those things are grueling for you:
a) Are you sure this is the right game for ya? ;)
b) Are you sure someone didn't already make a lib for that?
c) Outsource them!
Edit: If the main reason you want a co-founder is because you don't want to go it alone then you could fix that by joining a site like indi-hackers and talking to other founders as you work on your stuff. Maybe join a mastermind group. Etc.