Finding a developer
In board games, a developer is the person who takes a fundamentally working design and makes it publishable. Development is distinct from design: the designer invents the systems and finds the fun, while the developer tunes the numbers, tightens the rules language, cuts what does not earn its place, and runs the game through the volume of testing needed to make it bulletproof. At most publishers, an in-house or contract developer does this after a game is signed.
If you are self-publishing, that safety net does not exist, and hiring or recruiting development help is one of the smartest investments you can make. Freelance developers advertise in design communities, and experienced designers sometimes take development work between their own projects. Rates vary widely with scope, from a flat fee for a rules edit to a percentage or royalty share for full development of a game. Ask to see games they have developed and talk to the designers they worked with.
You can also get partial development for free by cultivating the right testers. A retired designer at your local group, the math-inclined regular who breaks every economy, the rules editor who rewrites your ambiguous sentences for fun: these people are doing development work. Credit them in the rulebook, and consider formalizing the relationship if one of them starts putting in serious hours.
Whatever the arrangement, the designer still owns the vision. A good developer proposes changes and explains the reasoning, then defers on what the game is trying to be. If someone wants to redesign your game rather than develop it, they are auditioning to be a co-designer, which is a different conversation with different terms.
Video tutorials
Finding the right mixture of luck and skill with Richard Garfield
How to design games that stand the test of time with Brad Talton