I've been dealing with this with clients recently too. Very annoying. All is not lost, however.
1. Any white-box clone shop can still make a computer for you, notebook or desktop, with Windows XP on it. In Southern California and other western states, PCClub isn't a bad place to go. They have internet sales too, so you can be anywhere and get a PC from them. I would imagine there are innumerable smaller computer chains that can build an XP box to suit.
2. From Dell, instead of clicking "Home/Home Office" as who you are, click "Small Business" or "Medium Business." Most, though not all, of those PCs can be had with either XP or Vista pre-installed for a small/medium business. The model numbers of the computers can be different, but the hardware is essentially the same. Just buy a good video card from Newegg and almost any of those "business machines" would be a good AH computer.
3. Call one of these vendors (Dell/Gateway) and tell the sales droid that you are prepared to buy a computer today, on this call, if it has XP on it. You never know...
4. Buy an Intel-based Mac. No, seriously. They don't come with Vista. Then install BootCamp and install XP. It is a nice, clean install that works just great.
5. Build your own, and install your own OS.
-Llama