The best method and quickest method in the game:
- Travelling between blacksmiths and buying olive wood
When travelling between backsmiths they will not always stock up, however, you can expect about 4 out of 5 to stock it and you'll generally get 100+ stock
best alternative method:
Not as efficient as trading.
Other ways to obtain olive wood:
Personally, when I was trading I would simply continue as normal completing quests and any blacksmith that I encountered or was "only slightly" out of the way would be where I bought my olive wood, although I'm guessing you recently hit level 25 which is why this is now your focus.