Ebay or other forum classifieds. Be prepared to pay $400-$500 for the intake, another $125 for the throttle cable and there are few other things too IIRC . You can also just use the Mark VIII intake.
Also, porting B heads is a waste of money and camming is another $1200+ just for the cams. If you do anything with the cams I would find an OEM set of 96-98 Cobra intake cams and swap them in. The exhaust cams will be the same but the 96-98 Cobra cams are the most aggressive OEM grind and a common upgrade for anyone with a 4V who doesn't want to drop $1200 on aftermarket stuff.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.