Toptal specializes in Web development and design, but I'm still surprised they couldn't offer an OS X resource.
Just keep in mind, that if you're looking for a "developer", then be prepared to manage the whole process yourself. This includes, but is not limited to planning, project management, creating a build environment, devops, QA, etc.
Depending on your budget, you can go with a (small) software agency that does a full-cycle development for you.
Speaking specifically of OS X development, there are many multi-platform tools that allow creating an app in one framework/language, and then port it to different operating systems with minimal effort. The downside is, such apps look ugly in one platform (or all of them :) )
Sometimes, an OS X "app" is a little more than well-packaged web application. See Slack app for example: https://itunes.apple.com/ca/app/slack/id803453959?mt=12
So in the end you might not need a "mac OS" developer, who is hard to find and who might be very expensive. You just need a clever strategy of how to package the functionality you need on OS X.