cut paste from 2d images..do a search on google.. ask for xtra large images. and cut paste.
If you printing billboard size..resample images before you cut paste. good idea to resample images to 300dpi and reduce the pix physical size.
Or use daz.. free..and all the poser chaps you can use.. good idea..then you can render it to the dpi and size you require..and then cut paste in photoshop.or do it in daz completely.
Billboards are not fun to do... curious question... are u using a vector based program..ie..illustrator or corel draw or indesign? makes life lots easier with one of these...