A map is available upon request.
Unnatural word-for-word translation.
would be much better.
First, if you say ご要望 in a polite way, you need お送りします. 送ります instead even implies that “I’m reluctant to do so, but as per your request, I’ll send the map to you”. That’s rude.
Second, ご要望により地図 sounds unnatural a bit and redundant. It should be ご要望の地図.
Even if you replace the original Japanese sentence ご要望により地図を送ります with ご要望の地図をお送りします, a big problem remains. These are more likely interpreted as “I am sending the map you requested”. This means that the ship has already got an order from a customer, and the sentence is written in an email as a shipment confirmation message.
But the English sentence is talking about the on-a-request-basis policy. ○○のご予約承ります is a common phrase for it.