While supply for Apple’s iPhone 5c has been strong enough to deter complete sell-outs from carrier stores and Apple’s own online store, sources say that supplies of the higher-end iPhone 5s will be constrained at launch (as we expected). According to sources at several large Apple Stores across the country, about 70-80% of new iPhones stocked for the launch on Friday, September 20th are the iPhone 5c.

The iPhone 5s will be available in limited quantity, and Apple is expecting the Space Gray model to be most popular, according to our sources. Of the iPhone 5s units scheduled for delivery to Apple Stores for the launch this Friday, about an average of three out of every four units will be a Space Gray model. The white/silver and white/gold options will be hard to come by at launch…

We’re also told to expect supply of AT&T models (in the United States) to be stronger than Verizon Wireless models in the higher-capacity configurations for the 5s. In terms of capacities, in general, we’re hearing that 16GB and 32GB configurations could be easier to find than the higher-end 64GB models. As BGR postulates, perhaps the iPhone 5s constraints are the reason behind the new iPhone pre-orders being exclusive to the iPhone 5c. AllThingsD is separately reporting that carrier stores are also seeing constraints.

Because supplies are expected to be constrained, Apple is said to be preparing an update to its Apple Store app for the iPhone and iPod touch that can allow customers to monitor if certain new iPhone models are available at their local Apple Store. The app is said to arrive by next week, but it, of course, could be pushed back or cancelled at the last minute.