IAP-Sample is a project mainly written in OBJECTIVE-C and C, it's free.
A basic example of how to do In-App Purchases in iOS
This is a really basic app that demonstrates the bare minimum you need to do to perform in-app purchases in iOS.
In InAppPurchasesManager.m, find this line:
SKProductsRequest *req = [[SKProductsRequest alloc] initWithProductIdentifiers:[NSSet setWithObjects:]];
Add your product identifiers (that you set up in iTunes Connect) to the NSSet.