There are, but that's very hard to find.
The main issue is finding a designer or "partner" who has the same goals as you. The most experienced designers will want an appropriate payment for their work and usually already have a bank of clients. They often don't need a portfolio either unless they want to change industry. The youngest ones are often unreliable, are full of short term spark at the start and need a lot of training on pretty much everything, including how to write a proper response by email. And on top of this, it's hard to evaluate what's the value of the coding work that was done versus the design work; if you exchange services, it needs to be fair on both sides.
Then there's the ego; that's a problem for everyone, at different degrees!
From experience, it's easier to hire people, pay them and keep the leader role. It's also easier to fire them and keep some control (and privacy) on your clients' details. Then later when things seem to work fine, a partnership can be created. Consider your offer in a similar way as community work and with similar results; people who work without payment don't feel like they'll owe you anything and won't feel they have responsibility towards the projects, at least not as much as someone who gets paid for it. You could meet community centers managers and ask them about the volunteers rotation, and how it's hard to retain them. You will go through the "enchanted" phase the first few weeks when you'll meet candidates then all the little bugs will appear and you'll realize the person is really not a good fit and maybe even cause you losses.
In most case, when you work with other people, everybody got their own self-interest. If the plan is to share design work to build a portfolio and if there's no money involved, expect people to get what they need and to see them go. Even with a pay check, it's often hard to work with some people since they have a... welfare attitude. If they're not used to make money (eg. new designers), they'll feel rich after cashing $500 and you'll only hear from them once that money got spent (teehee!) The same often happen with part-time freelancers who also have another job; they will not put priority on your projects and you'll always be a #2. If you are alright with this, then it's fine.
How to find good candidates? Hire them, consider them as employees or business relationships and don't get too friendly. Do interviews, find them in your network, do the same process as any other businesses. Hang around places like the InkSpot chat on this Stack or Facebook groups, observe people and then make an offer if you want to work with them. Don't use the formula "free work but full of potential"; that will really turn off the real good designers as they hear that pretty much e-v-e-r-y-d-a-y!
There's tons of good designers but you need to find the ones who have passion for design, motivation (eg. wants to make money), are not idealists and have good social or at least business skills. That's why observing people in your network or infiltrating designers' circles is the best way to find them... in my opinion.