Your best bet is to click on "Rankings" in the upper right, than look at town Rankings, there you can see a list of the biggest towns who are most likely to have a shop with what you are looking for. Check to see if they have the item, then click on the signpost near the bottom of the town screen to see how far a walk it is for you to see if it is within the range you would like. It will be a little work but it is the best way.
The only way to find if going town by town to see if they got the shop level to sell the item you are looking for... There will probably be a utility site soon that will help you find what you want on the map... although at the moment its not setup!