To search for just a single tag, the only way I could find is to go to one of the products in your shop and make a temporary tag that you want to search for and then just click on it when out of edit mode. When you're done searching you just go back to that product and delete the tag. Or you can find a shop that already has the tag and click on it.
Post up any search methods you come up with too if you want!
[Edit] Made changes to reflect changes to shop page format.