If you are having no luck locating a specific item from the regular shop or the traders you meet from your quests you may need to visit the Secret Shop which can be accessed at the bar on the main screen. It's always worth visiting the Secret Shop and checking what items it has for sale as there is always the possibility you will find a cheap item there that is difficult to acquire from other sources.
Whenever you upgrade the Secret Shop the merchandise it has will significantly improve. As long as you meet the account requirement you will be able to level up the Secret Shop by spending gold.
We have questions and answers related to this topic which may also help you: