This post is mostly about sharing what has and hasn't worked for me when I've tried finding a technical cofounder. I'm happy to give personalized advice based on your specific situation if you leave a comment explaining what you're working on and what you're looking for.
As quick background: I'm technical, but all the advice below applies equally to technical and non-technical founders. At the moment, I'm passively seeking a cofounder, and am mostly focused on implementing an MVP on my own. I'll restart my cofounder search more actively once I have some market validation and a bit of traction. (But if you're interested, feel free to reach out.) In the past I've invested a lot of time in finding a cofounder, have found a few, and have talked to a lot of other people about their cofounder search.
The first question I always ask someone when advising them on finding a technical cofounder is: Why do you need one right now? If you're nontechnical and don't have any market validation or traction yet, it can be challenging to find a technical cofounder, since they're often in high demand. If you have low-hanging fruit when it comes to finding a cofounder, such as a friend who would happily work with you, then wooing them can be a great investment of time. One of my near-misses was a coworker whom I respected and liked working with. I knew he was interested in starting a startup, and I talked to him about an idea I wanted to work on...but this was just as he was starting a new job, and he wanted to invest all his available free time in ramping up at that job. I'm still working on wooing him over the long term, though :)
Otherwise, the fastest way to a strong cofounder might be to validate your idea some first, so you and your startup hypothesis are more valuable. This is too large a topic to cover in any depth here, but two approaches to validating your idea quickly:
When you do go out looking for a cofounder, you'll be much likelier to entice someone if you can tell them you already have a waitlist of 150 people, or you have some early users of the MVP product you built using Airtable.
So how do you find a technical cofounder once you're ready?
If you haven't already signed up for Startup School's cofounder-matchmaking tool, I recommend it. In general, I'm skeptical of cofounder matchmaking sites, because I've had a poor experience with them. Any matchmaking app requires critical mass in order to be successful, and most of these sites don't have it - they're ghost towns. But Startup School already has a huge user base, so I'm a lot more hopeful that it will work out. I've had some good success with it so far. I've only been passively searching, but have already had several good conversations, and am about to work on a small pilot project with one founder.
I've had a little bit of success posting a "seeking cofounder" post in the Reddit cofounder subreddit. The successful posts I see (and my post, which was moderately successful) had a lot of detail about the project the founder wanted help with. At first glance, IndieHackers' cofounder forum looks livelier, but I haven't used it myself.
(If you end up writing a Startup School cofounder profile, or a Reddit/IH seeking-cofounder post, I'd be happy to review it.)
I've had good success using Lunchclub, a matchmaker for professional connections. It's free (well, freemium, but the free plan works well for me). They match you based on your interests. I listed my interests as "starting a company" and "seeking a cofounder", and in one of my first meetings I found a guy I ended up working with for a bit on an idea that ended up taking us to a YC interview. About three quarters of the meetings I have are very thought provoking and useful.
Participate in a hackathon (see listings) or startup pitch event (some examples). Even at hackathons, there's usually a mix of coders and non-coders. This could be a good way to find someone who wants to join your team outside the hackathon, but even inside the hackathon, it could be useful to get a proof of concept built.
Look up coding meetups (I'm sure there are some virtual ones), and attend some of them. Look for ones that have open discussion sections, not ones that are just lectures. There are inevitably some coders at events like this that are looking for an interesting side project to work on.
During quarantine, one caveat about the hackathon and meetup suggestions: I would be highly skeptical of any that are organized as a single, big zoom room. You want to be able to mingle and talk to people individually. Unless they use some platform like Gather.town that allows you to virtually walk around, or some sort of speed-dating format, then it's probably not going to be a useful way to find potential cofounders.
A friend said he knew someone who found coders by making job postings on LinkedIn. I imagine this involves creating an employer profile, then creating a job posting - maybe as an internship? I don't know details of how LinkedIn's job posting platform works, but it sounded like a promising idea to me. You could probably do the same on other job sites. Some or all of these probably charge to make postings, but some of the fees might not be very high. I know for a while Craigslist didn't charge very much for job postings.
When you're talking to a potential cofounder, how do you maximize your chances of getting them to work with you? As mentioned above, having some market validation or traction will go a long way. But what can you do in just a few minutes before a conversation to make the most of it? This is also a topic too big to cover adequately here, but a couple of thoughts:
If you're trying to find a cofounder, especially a technical cofounder, I'm interested in hearing about what you're going through. Let me know, and I'm happy to dig into details with you and help you come up with a plan!