How big of a xfmr do you need? That is a big part of this. It will be sized in kVA. If you know what you need.....Primary voltage and Secondary voltage, and kVA......call a supply house and they will order what you need.
Are you just trying to get it acquired so your electrician can install it when he returns, or are you looking to do this yourself?
No disrespect meant here....but if you don't know how to size/order a xfmr....you likely shouldn't be installing it. ESPECIALLY with 480V.