Check behind the seats, that's the usual spot for 90-2000s Astons. Lift the covers behind the seat and if you can see the battery isolator panel then the battery is behind the panel. I'm quite sure it will be in the owners manual for specific directions.
On the 90's Vantage the battery is in the boot left hand side the battery on/off switch is right next it. This is same location for the 70’s 80’s V8 plus Virage, V8 Coupe, LWB and the Vantage.
The fasteners on the battery cover just turn half a turn, four of them. They can be a real fiddle to get back in, especially the top right one. Just put an optima red top on my V550 tonight. Not tried it yet as I've got the starter motor off.