What are you shooting?
For people, the spectral sensitivity of tmax400 is different than tri-x and could be a bit different, but not insurmountable.
For landscape, I've seen many Tri-x photos that seem fairly interchangable with tmax 400.
I keep a stockpile of what I need for film. Your credit card might get stolen and you wouldn't be able to order, you could have a financial emergency and not be able to buy film. There could a be a storm or natural disaster or terror event preventing shipping.