Well you can make a custom one, i'm probably not the best to give advice on it as i never tried it before although i intend to at some point.
My plan is basically to take the top part of my bumper (with all the mounting points that attach it to the car) and then build up a 1 off bumper from other bits of bumper, now that could be just 1 bumper all cut up and stuck back together to fit or parts from a variety of different bumpers to give a full on one off look.
You should be able to get advice on what to actually use to bond it all together on here, i can remember seeing a few people on here doing similar stuff. I've just not looked at it in detail yet, i'm a long way off starting that yet