You can place ads looking for a writing partner. You can do it here.
The thing is, if there is to be a successful collaboration you both have to be on the "same page". You both have to want success in equal measure. You both have to be desperate to succeed. And, perhaps, you need to be complete opposites. He covers the parts that don't come natural to you, and vice versa. One is a volcano and the other is detail oriented.
It's this "opposites" factor that throws so many people off. People assume you have to be great friends in order to create great stuff together but I don't agree. You just have to share the same vision of success and have the same amount of drive & desperation to succeed.
I have advertised for a collaborator here at BSG a few times since June of 2007 but no takers. So I changed the ad to a sketch show group effort but attracted nothing but first draft garbage. The participants had no follow-up "drive". No real indications of wanting to make the group effort work out. They'd just send me their first draft drivel and then bugger off without any follow-up. No checking up to see how things were going. They just used the opportunity to send their first draft tit-wattle to the f**ken dumpster.
I'd reckon 98% of those here at BSG who claim to wanna make a living from their writing are either very scared to succeed or know they are nothing but dabblers and hobbyists of the weakest variety.
Do your research and you will find that most successful collaborations involve opposites.
But again, people are put off by that. They don't have the sense to know how fruitful such a collaboration can be. They figure if you come across personally as someone they'd rather not hang out with socially then there is no chance you could write well together.
Do your research. Most successful writing partners DO NOT hang out socially.