You will have to buy it. You can get an OEM version cheaper, but this is probably your best bet.
Newegg.com - Microsoft Windows XP Home with SP2 - Retail
If you need it much faster, you can probably still get it at Wal-Mart, Staples, Best Buy or any other store that sells electronics.
The only way to put XP on an already vista running computer is to erase the hard drive and install Windows XP from the beginning.