More details would be helpful. Are you looking for the 3rd brake light or the corner lights? What year/model is your van? You can find them on any of the websites below by digging through the parts lists. These folks have the best prices but some charge more for shipping & handling.
Seeing it's tough to break a 3rd brake light I'll assume you mean the corner ones. There's 2 plastic pieces that flip open revealing screws. They are black and on the assembly closes to the center of the van, you open the hatch to see them. After the screws are off the housing pops off.
Honda parts sites you can find with google or try a junk yard/Ebay:
Check out this how to video. Cheers.
You may not be able to pull the light housing out of the fender without prying. As mentioned in the video, cover a screw driver with cloth when you pry the housing from the fender.
A search on google for "tail light housing 2003 honda odyssey" will give many where to buy listings. http://www.youtube.com/watch?v=IHlwt7Mrx00
I just realized that i never mentioned that I needed to replace the brake light lens and not the bulb. The video is great but it shows the bulb replacement not the lens cover. Once you remove the brake light unit, can the lens be removed too?