What "temp folder"???
I'm guessing that you mean you've removed one of the items in the Sidebar?
The Finder Sidebar does not actually contain any folders; it is merely a list of
"shortcuts" to the real folders.
As such, removing an item from the Sidebar only removes the "shortcut";
it does
not delete or move the
real folder to the Trash.
Here are the default "shortcuts" of the Sidebar:
At the top, above the separator:
-any drive that is currently connected (e.g., the hard drive, CDs, DVDs, USB drives)
-Network
If you removed any of these and want to re-add them,
go to Finder menu > Preferences...
In the Sidebar section, recheck the appropriate boxes.
After the separator:
-[your account name] (Home folder)
-Applications
-Documents
-Music
-Pictures
-Movies
-Desktop
These are just regular "shortcuts" to folders.
To re-add these, just go to the folder and drag it into the Sidebar.