Guild Wars 2

Hi , have a simple question>

The big backpack that the engineer starts out with , also known as universal multitool something..

is it available for other classes as well? in case, which ones, where do u get it?

Or is there any other backpacks visible in game on the characters back that you can get for other classes later on?

There are backpacks that represent your guild available from Guild Armorers.
http://wiki.guildwars2.com/wiki/Guild_Armorer

Engineer has a unique one, I’m sure. There are guild backpacks like Yashiro said, and if you were here for Wintersday, there’s the toy bag backpack
The only other visible back items are the fractal relic (something like that, looks nothing like a backpack), a quiver, and a book