If you truly want to go that far back, you're going to have to pay a premium price, because it's not being made or sold by Microsoft anymore, and because there is a limited supply now (and apparently there are still some buyers), the sellers are selling it for $150 - $300. To find those sellers, you can use Google to search for "Windows XP Media Center 2005".
I have found one--and only one--on http://www.bizrate.com/windows-xp-media-center-2005/ which link I gave in the last post I made here (except that this one is for Media Center 2005). That store is a store called, PC-Deal4U, and they say that theirs is regular priced $149.99 but it's on sale for only $79.99, but I wouldn't recommend that you buy from them.
CLUE: The method of delivery is DOWNLOADING, and Microsoft does NOT allow their products to be sold by downloading. They are adamant that you get the Genuine Product in a factory sealed box, complete with a genuine license key and hologram and all that special packaging stuff that they provide you to prove that it IS genuine.
If you don't get all that, you may end up spending the $79.99 for something that APPEARS to be working--but the first time you log onto the Internet and try to download Windows Updates, it'll tell you that it didn't pass the Genuine Advantage Notification, and I've seen cases where the computer was jammed so badly the only thing I could do to save the hard drive was to take it out of the computer, hook it up as a slave, and use a disk utility that works in non-Windows mode to WIPE the drive completely, just to make it usable again--and all the data, of course, was lost.
When Windows was running, even though it was setup as a SLAVE DRIVE, it wouldn't allow me to access anything. That's why I had to wipe it clean. The lady didn't want to pay for a new copy of Windows for such an old computer and that is the only way that Microsoft would give us the key to open it again.
Here is the Microsoft website that explains about the Genuine Advantage Notifications:
and this explains how you KNOW that it IS an authentic product:
BOTTOM LINE: Be very careful of what you buy!