It's hard to beat these guys' price if you're just looking for a straight pipe system.
eBay Motors: 1999 - 2003 Ford 7.3L 4" Straight Exhaust No Muffler (item 150209134758 end time Feb-01-08 18:38:37 PST)
eBay Motors: Ford 7.3L EXCURSION 4" Turbo Diesel Exhaust 99-Present (item 330204547015 end time Jan-25-08 07:55:30 PST)
But you can probably get one of the vendors on here to get you a better deal than that. Or you could just do a bigger downpipe and a muffler delete pipe for about $100