Where to buy this transformer? Anyone?

Im looking for a 25-0-25, 200VA transformer with a 12V auxillary winding. Im not sure how the 12V auxillary winding works. If any of you experts know, please let me know. Is it something I have to add on to the transformer? I've been researching what the 25-0-25 means and it looks like its a full-wave transformer...? Anywho, anyone that can give me any help with this just reply to this.