Unluckily, there isn’t any third-party app available on the official App Store for locking apps on iPhone. This may be because of the iOS restrictions that give very limited permissions and access to third-party apps when it comes to security. If you have jailbroken your iPhone, there are plenty of apps that you can use such as BioLockDown, AppLocker, BioProtect, etc. These apps can be downloaded from Cydia. It’s not worth jailbreaking your iPhone just for using these applications.