You said you're concerned about how much it might cost. Surely you can afford to try someone out for a short amount of time, given the app makes $10K a year. If you shut it down or open source it, that income's gone anyway. So why not try to keep a good product alive?
One good place to look for a programmer might be the monthly HN threads by whoishiring - https://news.ycombinator.com/submitted?id=whoishiring