Be Prepared to Pre-Order
Unlike the iPhone, new iPads rarely sell out. That said, this year's iPad Pro appears to be a major upgrade from the previous models so there could be more demand than usual.
There's a good chance the new iPads will go up for pre-order at Midnight ahead of their release date. If Apple releases them in September, we could see them launch alongside the new iPhones.
If Apple announces the new iPad Pro models on September 12th like we think it will, we could see pre-orders start on September 14th with a release on September 21st.
If you want a very specific iPad Pro model (color, storage) we recommend staying up late just to make sure you get what you want.
Stock issues usually get solved within a few weeks so if you can't decide one a specific model ahead of time, make sure you take your time.