If you have never done any kind of programming before, there is a pretty long timescale to learn how to make a good app from scratch. I wouldn't want to discourage you from it, though, coding is a lot of fun. You will probably need some books, go through online tutorials, etc... You also need an iOS developer account which is $99/year.
On the other hand, it is not cheap to hire a good developer.
What do you mean by "purchase source code that is already created"? If there is someone selling an ecosystem that works for your needs (test prep) and then you just add your data and deploy the app, that would certainly be the quickest option for you.
Another option would be to code the app in HTML/CSS/Javascript and deploy it to iPhone AND Android AND Windows Phone via a tool like phonegap.