It is possible but not easy. In trying to purchase one, you might get ripped off. This is because Windows XP (although there will be security patches issued for two more years) is considered a discontinued operating system.
How many keys do you want to purchase? Do you want Retail keys or OEM keys or some kind of combination of each?
For an OEM key, you would need to purchase an acompanying unused OEM Windows XP Pro installation CD from a reputable vendor.
For a Retail key, you would need to purchase an accompanying Retail Windows XP Pro installation CD (either used or unused) from a reputable vendor.
These are not easy to find currently!