Monster Hunter

It depends, what specific items are you looking for? Is there just a couple, are there a ton, just a few, etc. I would reccomend checking the Item Location FAQ; it tells you where each item can be found.

Yessuh

Since there isn't any more information forthcoming, I will have to be specific. Almost any item that is HR13+ cannot be found offline with the notable exception of Rathalos and Rathian plates from Troublesome Pair (which are almost worthless offline only). The vast majority of things found online from HR1-12 can be found offline, possibly with a couple exceptions. You can get a couple Lao items offline through trades (find a Forest Elder trade list for more details, involves Rathalos and Rathian plates), but they cannot be used to create any Lao armor or weapons. The most notable online only items that are commonly used are Dragonite Ore and Union Ore. Unfortunately, this means many of the best weapons are restricted to those who have played online. So in general, assume that online-only items are un-obtainable offline.

Sm Lobstershell (plesioth) (1%)
Rathalos wing (1%)
Majestic Horn (1%)
Ruststone