Try looking on Newegg. I got my XP installation disk for Bootcamp 3 years ago. It was being sold as the OEM disk that is included with your typical PC. I'm not sure if they still have it but I don't think I paid more than $70 bucks or so. I also have no idea what a copy of XP will run you now, since Vista and now Windows 7 have come along, but it's worth a look.
I think the ones with your dell should work, because I also tried using the disks that came with my sister's HP. You just might need to find a new registration code somewhere.