Is it under copyright protection? If it’s someone’s art-- someone has the rights to that art, unless it’s public domain imagery where the copyright has expired, or the image was created with a declared intent for it to be freely used. So if there’s a legitimate source for your poster, it would be best to buy one.
If it is a public domain image, you’ll want to be careful about the resolution (stuff on the internet is usually 72 dpi, but stuff meant for print ought to be 300 dpi minimum).
There are computer programs that will enlarge images with minimal loss of quality, but the best large images are going to have a high-resolution source material, because the files were intended to be (a) printed in ink on paper, and (b) at a large scale.