Currently if you want to just get a white back strap (that has no stats or skin) you have to buy a backpack set that includes it. Please make back straps available as a separate item.
Why would you want a back strap? With the various different back slot skins now available to characters, if you want to move a back item skin to another character the only way you’re able to do so is by using transmutation stones and a back strap because every other back item is at least green (masterwork) quality and so becomes soulbound when transmuted.
http://dulfy.net/2013/06/27/gw2-backpiece-gallery/