Which one?
There is a flat 3 hole flange that goes between the up-pipe and the turbo (that always breaks), and then there is the flange that holds the up-pipe to the turbine side housing.
You may try Gbodyparts.com for the 3-holed flange. You'd have to call him though.
The flange between the up-pipe and the housing is hard to find. The other should be available at a local muffler shop.
Jerry