Where to get or find back trinkets?
I think I have only ever got a back slot item from personal story rewards.
My understanding is that you’ll get two through doing your personal story quest line. One at about level 35 and one at level cap.
Waiting For Death [WFD]
@ Borlis Pass Server
In addition to getting the “Back” slot accessories from your personal story, you can buy some from the Guild Armor vendor.
I think the level 30 one is 1g, level 60 one is 3g, and level 80 one is 5g if you buy them from the Guild Armorer.
There’s a little bit more information on the back items listed in their tool tips before you buy them.
I know the level 80 equip appears on your character as a backpack complete with guild logo.
Currently, there are only 3 ways to getting someone there, two of which have been listed.
Personal Story reward (twice, one ’round 30, 35 or so; one at the level cap)
Guild Armorer (requires being in a guild with the Armorer being unlocked)
Be an Engineer and picking the backpack option at character creation.
Capes and other back slots are planned future additions, reportedly. For capes there were clipping issue that delayed their release.
Huh, pretty sure I did not get one in the 30 bracket with my guardian.
“Whose Charr is this?”- “Ted’s.”
“Who’s Ted?”- “Ted’s dead, baby. Ted’s dead.”
Huh, pretty sure I did not get one in the 30 bracket with my guardian.
IIRC, you are given three versions from which to choose at the end of the level 35ish personal story chapter, each with a different set of stats.
Waiting For Death [WFD]
@ Borlis Pass Server
Can’t edit my post to add this, but here is a database link to the item I selected from my level 33 personal story quest line:
http://www.gw2db.com/items/59957-strong-back-brace
The wiki offers some additional info:
Waiting For Death [WFD]
@ Borlis Pass Server
Lol – thanks for the link – I did not read it properly, thought it was an upgrade item that can be upgraded itself – hehe. Gotta equip it next time I’m on right away.
Talk about a duh-moment. 
“Whose Charr is this?”- “Ted’s.”
“Who’s Ted?”- “Ted’s dead, baby. Ted’s dead.”