There are a few times in Episerver when you need to know a page types Id. In Episerver 6 you use the GetPageTypeID-method that return the Id. In Episerver 7 and upwards this has moved. To access the code now you can use the IContentTypeRepository.
In IContentTypeRepository you have useful features like Load<>, Save<>, Delete<> and List() to get a list of all page types in your website.
var pageTypeId = ServiceLocator.Current.GetInstance<IContentTypeRepository>() .Load<PageType>() .ID;