the sticker paper may found in the computer product supply shop....maybe cost few dollars for one A4 size...
but as i know.....the paper is for lazer printer to get a better quality.......for the jet printer.....i'm not
sure how good the end product is.....
for the water proof issue.....many guys will put one more piece of transparent sticker on the top of the image to protect it....as the guys said it won't be too thick la!!!hope can helped!!
Please note that if you use inkjet to print decal, unless the ink are of very high quality, (photo-link? ), the printed decal will be de-coloured after some time due to direct exploure to sun-light.