Its amazing when you type into Google - Rodeo and animal welfare - how much you get from Rodeo associations etc defending it!
I would also have started with HSUS but since you've got that already here are a couple of links that might be good.
Interview with a vet who turned away from rodeo:
http://www.sharkonline.org/?P=0000000480
Peta's factsheet (but no good refs):
http://www.peta.org/factsheet/files/FactsheetDisplay.asp?ID=69
Wikipedia has a bit about animal cruelty on their rodeo page:
http://en.wikipedia.org/wiki/Rodeo
Haven't managed to find any good solid info sites on animal welfare rodeo concerns though.
Great that you're writing a letter though, really can make a difference (I work for WSPA and we follow-up supporter's letters with organising interventions etc when we can, or forward them to someone who can do something)
Suz