off road lighting

what is the largest diameter of off road light is there? i keep finding 6" and 8" ones, but i have seen larger ones on rally, endurance race and baja cars and trucks. where do they get them?

I found these 9.5" and it's about the biggest that you'll get.
http://www.off-roadlights.com/
click on shopping cart and click on driving lights

thanks. that looks about what i might be looking for.