I usually buy from the official store, shipping is fast, usually 2-3 days if you pay express, other than that there is a store called Anfield Shop bases on Seattle that also sells LFC merchandise.
If you go to sites like Amazon, Fanatics, Dick's, etc. don't think there is much LFC stuff there and definitely there isn't anything in physical stores (I've been to a couple NB stores).
Pretty sure with Nike, getting LFC merchandise in the States (and other places too like Latin America where I'm from) and online is going to be much much easier.
For example, in Costa Rica the Adidas and Nike stores sell the jerseys of the teams they sponsor (bigger ones of course) go to a MB store and it's only running stuff that they sell.