What exactly are Bamboo shoots?
Bamboo isn't a tree, it's a grass ;)

A bamboo shoot is just the budding new bamboo that's harvested before it grows and becomes hard and stringy.

Bamboo shoots are generally available in 2 forms, fresh and canned. 
Fresh ones are sold whole and generally used in stir fries.

Canned bamboo shoots are precooked and packed in water. You can find canned bamboo shoots sold whole, shredded, or sliced. For hot and sour soup (which I assume you're making) you'll probably want the shredded ones.

There are also jarred bamboo shoots. You probably don't want these as they are often fermented and add different seasonings to the shoots.

I'm not sure where you are so I don't know how available they would be, but you can generally find canned ones in the asian section of major supermarkets.  Fresh ones are a little harder to come by and I believe only seasonal