Moodymoose, you do yourself an injustice saying you have no writing skills. Your post made me smile and put your enthusiasm across very nicely with a dash of humour. If you use the same style in your letter you should get some good results.
Do you need to write to companies for sponsorship? I've never heard of a bull roast before (about the biggest things we roast here are hogs but that sounds great!) but perhaps you could write to local companies explaining what you are doing and giving them the option to sponsor a 'leg' so so get 4 sponsors to cover the cost of the bull. You'll have to write to a lot more than just 4 though, if you are lucky enough to get more than 4 responses then you could contact the 'losers' and see if they would like to sponsor something else, come up with a few other fun options. You;ll have to offer them something in return - perhaps some free tickets to the event and of course some publicity (photo of the boss taken with the roast bull, space to put up an advertising banner, mentioning sponsors in programme/on a signboard.)