Remember that you purchase a license - the right to use the software - you do NOT purchase or own the software itself. So, you should already have the license - you just need the installation media. Microsoft softened their approach to activation in V/7/2008/2008R2, allowing an OEM key to activate Retail media, but that was not the case with 2003 ... that is Microsoft's issue, not Dell's.
Microsoft sets the rules for OEM's for things such as media distribution, and while yes, it is Dell's responsibility to distribute the media, how long is feasible to stock it? Should I be able to call Dell and request Windows 98 media for my first computer? Sure, it would be nice if they did, but Microsoft does not require that of OEM's (and their policy likely already adheres to any applicable laws), so likely none do it voluntarily as simply a favor to the customer. Dell stopped stocking 2003 media 4 years ago, and it isn't just them.
Also, Dell doesn't distribute authentic retail Microsoft OS media (hologram), and if they did, it would come with a Retail key, and there wouldn't be an issue activating. Dell's media is Dell-branded/labeled OEM Windows media.
So, if the media has been lost, you just need to find it ... partner or neighboring companies, local tech firms, schools, or the likes of eBay might be the best options for obtaining the correct media for the license you likely have (OEM).