Try eBay, if Microsoft or shops doesn't have them anymore.

But since you already reloaded XP (that's what you write), all you might need to download is the SP3 update. That's only 500 MB. That's 5 to 10 minutes if you can do it with a friend of family member or work where they have broadband. Then you can install it from your USB-stick.
I see no need for a CD then.

Kees