get the measurements, cut a bit of paper to scale the right size. Go to a carpet place and look at off cuts. Get one that will just about fit with a bit over. Then find someone out the local paper who will fit it for you on the cheap. I have fitted carpets myself, but not one for a room the size you mention cath. I did carpet a bathroom once!!! I made a template and cut it all out (roughly) and then just made it fit properly when it was in place. I almost got it right, but ended up with a bit too short round the bog! It was ok though.
Good luck :)