Did he have such a history of apprentices getting their legs chopped off and being so badly burned at the same time they'd need a mask, that he had a stack of these outfits just waiting to go?
Or can we consider this a possible plot hole?
Palpatine obviously has so much resources at his disposal, that he can do anything. Having a Vader suit is obviously a pre-caution to extensive battle damage for an appreciate/fellow Sith. Who knows, if Darth Maul, or even Count Dooku were seriously injured and not died, either of them would've been fitted with a Vader or similiar suit. However, that's neither here not there.
He had lots of resources available to him. Maybe he had a prototype and had a production version built for Anakin. Maybe he had it on hand in case he himself needed it.
He also has some ability to see the future, so maybe he foresaw that he needed it.