Kacky
Noah's Ark is popular so you may find them in your local craft or decorating store. Or you can buy online. A google search turned up quite a few. here is one And Wallies makes some nice stick-ups = .

quornandwafflesagain
have an existing picture enlarged on a photo copier, you can even get it laminated and put pictures up, you can copy a picture by dividing into sections and copy one section at a time. for a childs room the more naive the picture the better, be bold with your colours and just go for it

tmnasn
Check out clipart images on the net. Theres bound to be some you can print out.