What level are you? Remember, no one can wear any armor until level 22. If you're 22 or higher, you can contact an armorsmith directly, or do a vendor search on the bazaars. Click Galaxywide and Vendor Search. The easiest, actually, is to search under Misc/Wearable Containers and in the text search box, type "Imperial" - that should call up full suits of armor that you're looking for. Not until level 22, though, don't forget. (You can buy it sooner, but you can't wear it until 22.) If you're really looking for an Imp costume before that, you can always become an Imp pilot and run around in your TIE flight suit once you get it.