Ceramic tile sizes

i have recently moved into a new propoerty and the builders have only tiled half way up the bathroom wall above the bath. I want to fit a shower and am extending the tiling to the ceiling. I have measured the current tiles which are 146mm x 146mm, imperial is 5-3/4" (plain white cheap ones). I have been to B&Q and Homebase but they only have 150mmx150mm. I have looked on a few websites but had no luck either. Does anyone know where on earth can i get tiles the same size?

Would it be totally beyond reasoning to call the builders and ask them where they got the tiles from?

that was my plan for Monday, I just wanted to get it done this weekend and thought i could get them tomorrow. my other plan is to put a border up and do a diagonal pattern at the top?