There isn't currently a way to directly (through a supported api call) get the frame of a specific tab bar item. What I do is just show the popup from the left end of the tab bar like this:
CGFloat tabBarHeight = self.tabBarController.tabBar.bounds.size.height;
CGRect rect = CGRectMake(0, 0, tabBarHeight, tabBarHeight);
[popoverController presentPopoverFromRect:rect
inView:self.tabBarController.tabBar
permittedArrowDirections:UIPopoverArrowDirectionDown animated:YES];
You could try to estimate the position of the tab bar item taking the current orientation into account if you really wanted to and adjust the X coordinate of the rect above.
You could also look through the subviews of the tabbar and find the UITabBarButton object but that's not documented so it's not recommended.
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