Search the trade post for "spineguard".
Back pieces drop fairly often from Fractals of the Mists, in anywhere from masterwork to exotic quality. There are five different stat combinations, and only two of the guild backpacks' stats are available as drops. The Exotics are account bound, but the rares can be put up for sell on the trading post, and they're cheap.
At least, compared to the five gold you have to pay per guild backpack. I hold no illusion that someone isn't going to capitalize on that, but at the moment many of them are selling for five silver each. The value on them if you're looking for a rare back piece while you search for exotics or ascended is incredible.
So if you need a back piece for cheap, this is the best time to get one. You won't be able to display a guild emblem, but for most people that's not going to be a problem.
edit2: gogo gadget downvotes for being nice
Category: Reddit Guildwars2