If you want work done, you need to pay for it
It's not news that if you want work done you usually need to pay people to do it; there are not that many selfless volunteers and sooner or later you stop being able to trick people into doing it for free. Of course, the open source movement is a clear demonstration that you don't have to pay people in money; people find any number of things rewarding. But you do need to give people a reward that they find meaningful.
(You can argue that a certain amount of open source's problems come when its lack of money means that it doesn't have adequate rewards for certain sorts of work. Any time you hear something called 'thankless', it is a good tipoff that no adequate reward for it exists.)
The more time the work will take, the higher the odds are that you will have to pay real money in order to get it done. Setting aside the speed at which you get results (perhaps you're very patient), there's a big difference between how many people are willing to devote an hour of spare time to something and how many people are willing to devote a thousand hours of spare time to something.