We've had 2 ponies with sarcoids and used bloodroot cream on them both with great success. It isn't as nasty as Liverpool cream and is a fraction of the cost, especially as you can apply it yourself. You can get it on Ebay. It is a bit caustic, so needs to be used with care, but it doesn't seem to be unduly painful to the pony, unlike Liverpool cream, from what I hear.
Funnily enough, I was reading about using copper sulphate on sarcoids just a few days ago. Some people swear by it, it seems!