Strictly speaking, you've created a tableview cell that displays several values. Sitting in memory somewhere behind that is an array that contains the actual data. That's how MVC architecture works.
Ideally, you have some data structure in memory that corresponds to the cells. If you had an NSMutableArray in memory, you could retrieve the value by doing
// assumes one section in the table view
NSString* sourceForNextQuery = [dataSourceUsedToPopulate objectAtIndex: indexPath.row];
inside didSelectRowAtIndex
If you insist on retrieving the text from the label in the table view cell, when you create the label that contains it, add a tag (mySourceLabel.tag = 999). Then find that tag again by using searching for the tag, the get the text (UILabel.text)
But really, you want to separate the presentation of the data from the underlying model, and have the controller contain the data.