Its easy, go to your local commercial vehicle or lorry spares place.

Bet theres one nearby in an inductrial estate, check the yellow pages.

I had to do this in the past, only cost a coupla quid, stainless too.

I remember thinking "where the FUCK am i gonna find clamps that big" but then remembered all big vans and lorries and busses and shit all have 3inch+ exhausts.