There are a couple of options. These options are visiting meetups, startup events, using your network and finding them online. I will give you a quick list of sites that you can use to find your business partner.

Founder2be - Founder2be is a website that allows both startups and individuals to find one another. Hence, it allows their users to find co-founders. Users can specify the country and position they are looking for. Then, they can scroll through individuals that match that criteria.

Founders Nation - Very similar to Founder2be, this site allows users to look for people that are interested in joining startups. Each person has a profile that states their skills, what they are looking for/what they want to do, their stage, contact details, location and much more.

Work in Startups - A job board listing for startups and startup jobs. There is a whole section for startups looking for co-founders. The main focus of the site is for people looking to join businesses. Hence, it would be a good idea to advertise the position on this site.

Angel List - Similar to Work In Startups, Angel list allows their users to list their company and then advertise hiring positions. Furthermore, Angel List can be useful if you are ever interested in looking for investors. The main focus of this site is to get people that have the same passion for startups together.
Like I said earlier, don’t forget to visit meetups and startup events as well. The face to face contact can be extremely useful for finding your business partner!

I hope this helps!