I have had trouble finding those in the past as well. Muffler shops don't know what to do for them, parts stores don't sell them. I always make sure I have several inches of the down pipe whenever I get exhaust manifolds off something. It's easier to take a sawzall to the pipe anyway.
I would suggest hitting a salvage yard for those. They may even just give them to you. Just make sure you get the right diameter pipe, and get the 3 bolt flanges too if you need 'em. A few minutes in a blast cabinet usually cleans them up nice enough to use. Parts stores usually carry studs and the good brass nuts.
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