ebay has plenty
http://shop.ebay.co.uk/i.html?rt=nc&...=p3286.c0.m283
Try and make sure it has at least SP1 installed, support for XP without a service pack ended about a year ago, and it saves you the trouble of downloading the standalone service packs from MS to install after.
Personally, for the XP machine I used to run I downloaded the standalone service packs, slipstreamed them and burned additional copies plus kept the ISO on a portable drive so if I needed the CD I could simply burn a copy if I couldn't find one. The product you install is licensed to the CoA not the media you install it from.