I had an exahust leak like that on a car I just needed fixed.
went to autozone and picked up an exhaust leak patch kit basically just came with puddy, I filled the gap and all along the flange with the puddy let it dry and called it a day $10 fix... perfect if you just need it done quick.
if you want to be a bit more pro you could try a "Remflex" gasket the suckers are thick and they can compress quite a bit, more than enough to fill your gap IMO... though you have to order them online so that means waiting for shipping... and they're like $8 a gasket (but worth it IMO) they are reusable though...
http://www.remflex.com/