Just trying to help – if they’re not supplying the IT, then presumably they’re not actually checking over her shoulder to see what she’s using, just interested in the format of what she sends them. Of course it’s far easier for them to say “use W2003” than “supply documents in W2003 format” which could easily be misinterpreted. I’ve edited files originally generated on W2003 on my W2007 installation before re-opening them on W2003 and it’s completely seamless – if they don’t check over her shoulder and don’t want her to do something obscure where there’s some strange difference they’ll never know.
Though if you want to spend more on W2003 despite the fact something available cheaper can do exactly the same job, feel free. As to an answer to the question you’re asking, I doubt anywhere sells it cheap, as there’s no demand outside companies using it for legacy reasons.