Does anyone know where I can purchase a mold of a bull for a rodeo cake I want to make the bull out of chocolate.

What size? Does it have to be 3D? You could get a bull figurine and make your own mold with, for example, Amazing Mold Putty.

I found this; looks like he's sitting down:

http://www.ebay.com/itm/3-MINIATURE-STUFFED-BULL-great-detail-never-poured-Nowell-Mold-NM627-OCF-1009-/140958889561?pt=LH_DefaultDomain_0&hash=item20d1cdf659

Teeny weeny:

http://www.ebay.com/itm/Bull-Push-Mold-Gum-Paste-Food-Safe-Silicone-884-Cake-Chocolate-Resin-Clay-/221206883487?pt=LH_DefaultDomain_0&hash=item3380f52c9f

Larger, not 3D:

http://www.ebay.com/itm/plaster-cement-Bull-plastic-travertine-tile-mold-/400005094219?pt=LH_DefaultDomain_0&hash=item5d22295b4b