Your camera won't be able to handle a high iso without being extremely grainy and destroying your photo. What you need is a faster lens, a minimum of a f2.8 preferably faster. A flash won't work at the distance your shooting. I would suggest a Cannon 70-200mm f2.8. It is quite pricey to buy but many photo shops allow you to rent a lens for a fairly decent amount. I believe that that lens is about $25/day on average. And well worth the money. You will still have to bump up your ISO a bit but not nearly as much as you would without the faster lens. Would be a difference of an ISO of about 320-400 or 1600 or more.
The wind of heaven is that which blows between a horse's ears. ~Arabian Proverb