I have checked every store in LA and can't find me a 16gb space gray T-Mobile iPhone SE. Anywhere. I have even called a crap ton of T-Mobile Stores.
Well, that was at least 3 weeks ago when I first tried to buy my 64GB SE. So its entirely likely now that even the 16GB crowd demand has caught up with Apple's underestimated supply. When the Apple Store didn't have the 64GB in stock, the sales associate tried to get me to buy one of the many 16GB he had in stock, which after I refused he admitted nobody really wanted -- but they were flush with them at the time.
If you are paying cash upfront, there's a 16GB Space Gray ATT model in stock at almost every Apple Store in LA, which will be sold to you unlocked and allow you to put your T-Mobile Sim into it. Exact same phone.