Ebay / Craigslist / local buy and sell pages are good places to keep an eye on,along with checking with the local auto wreckers/SGI Salvage etc. You can buy them new through Matt Fikac at Moes or MMX,they both sell them in stock and ported form,but they'll make your butt pucker on price by the time you get one shipped to your front door in Canada.
They're a composite manifold,so they're easy to port at home with a dremel and shaft attachment,just a bit time consuming to do,especially if you're a bit aanal like me.I haven't ported the 6.4 truck manifold,but i have ported the stock 5.7 truck intake and the stock 6.4 manifold on the wifes car, taking my time i usually figure on a couple evenings work to port one