I wouldn't say there are poster templates, but you can find backgrounds (graphic and photo) in a few places. That will get you started, then you really just need a good collections of fonts. But no, there is not easy button for posters, etc. You just have to know how to use design software, and understand good layout, color, etc.
I'm assuming you're on a budget so here's some almost free, and pay options for you.
royalty free photography...
stock.xchng - the leading free stock photography site
pay attention to restrictions, most people just want to see what you used their stuff on. do a search for textures, etc and you can find a good starting point for backgrounds.
free vector art..
Free Vector Art - Download Free Vector Art Graphics at Vecteezy!
you'll need illustrator or photoshop to open these files, preferably illustrator so you can more easily manipulate elements. again, pay attention to any restrictions. I use pieces and parts from filigree and other small design elements, but rarely ever flat out use other peoples full BGs.
fonts...
dafont.com
most are free for personal use, pay attention to restrictions
then there's
Stock Photography: Search Royalty Free Images & Photos | iStockphoto.com, which you will probably find more stuff on. this is a pay site and the license agreement stuff is pretty black and white. add the art to a shopping cart, then pick between two licenses. most art is around $5 to $10 per piece.
again, these are just resources to help build posters from various elements, but there's no real poster template site out there. you just gotta know a little bit about graphic design.