It is quite annoying that you are not allowed to use the shop wizard when you are on a quest. You have to try your luck with the shop galleries to find the items you are looking for and if you are lucky you will find it, but at what price? Sometimes the items in the gallery are heavily over priced than the shop wizard. The trading post also requires luck. Some quests have to be completed within a short period of time like within 15 minutes or so.
Chances are that you will not get your item within the time limit from the trading post. There are several forums apparently where people help you out by searching the user names of people who have the item and give it to you but I have never bothered with doing that as it is too time consuming for me. I am sure you will agree with me that we all are here for an easy way out in what ever we do.