Investors invest in traction. If your app is almost done, then do whatever it takes to finish it and launch it.
The key number that investors want to see is growth greater than 10% per week, or 30% per month. Whatever your key metric is (users, traffic, messages exchaned, etc.).
Try crowdfunding on IndieGoGo. If you can't raise money from suckers, you'll never be able to raise money from seasoned investors.
If you are a developer, why don't you just finish it yourself? If you don't know how, why don't you just save money then hire an expert on ODesk?
If you want to invest in the mobile application then there are so many options available, Nowadays so many application like e-commerce app development, travel app development, a restaurant app development, a dating app development, Education app development, etc.
"Did you know that Tinder makes nearly #1500 per minute from users trying to connect with others." Dating apps are the best way to make an ample amount of as the competition in other sectors is high.
At this stage, you could consider raising a friends and family round. Once you have traction, you could consider raising an angel round.