If you're just working on an MVP you can use a freelancer sight like Upwork, but you have to be careful in your hiring.
1) Post a job stating everyone that applies should give you their github / bitbucket username and Trello username.
2) For the top 5 most promising people to apply, give them a simple task which should take about 30 - 60 min for a decent developer to do (e.g. implement this screen with these one or two actions).
3) Pick the developer that actually delivers exactly what you asked for the fastest, and in the most professional manner (not asking a million questions, etc.).
To assign and track this and subsequent tasks use a Trello board with three columns:
Ask the developer(s) to update the Trello cards in real time so you can keep track of what they're working on and for how long.
If you'd like any more specific advice in relation to your specific app idea let me know,
Are you saying that you want to buy out existing apps that may not be positioned well?
I am an app developer, but recently to get an app built (React-Native), I found it easier to break down the requirements into a technical architecture so it was clear what the specific components are. I then hired two separate developers to do the scaffolding work and got about 80% of the way within two days. It was also defined pretty generically so that I could combine the discrete features on my own and protect the IP.
Hope that helps.
I acquired my first monetized app from the original developer in October last year.
There are a ton of app and website brokerages out there, with a wide range of quality available - be prepared to kiss a lot of frogs before you find your Prince Charming!
I originally purchased my app business through Empire Flippers (empireflippers.com), and couldn't have been happier with their process and quality, but they've been gradually upping their minimum asking price for listings - I think they currently stand at ~$25k right now. Flippa.com is probably the biggest and best-known brokerage out there - I've used them several times from the sellers' side. They have tons of apps for sale at ~$5k (and lower), but tread carefully and do your due diligence: there are tons of scam and copycat apps out there by sellers who intend to take your money and leave you with a lemon. QuietLight and FE International are also fairly well-known website/app brokerages, although I can't comment as to the quality of their listings, not having personal experience with them.
You can also approach it the other way, by searching for the kind of apps you want to acquire, and contacting the developers directly to ask them whether they'd consider selling.
Hope that helps, and congrats in advance, the journey you're about to embark on is a really exciting, educational (and humbling) experience! =) I love talking about this stuff, and am happy to answer more questions, so feel free to set up a call, if you're interested. ~Jason