I'm 6' 4" and I do pretty much all my shopping online:
T-shirts/Long sleeves: J. Crew - Large tall (website only)
Button downs: Brooks Brothers luxury slim fit, Kent Wang MTM (J. Crew also has some large-tall ones that aren't bad one sale, but the collars are crappy)
I feel like the other categories are easier, but if you have any specific categories/items you are having difficulty finding, feel free.