The Optimas can be found pretty much at any auto store. I find the best price usually at Autozone. Just make sure you get the right ones. the only stipulation is they are not as large amp capacity as some of the bigger lead acids so they may not work in every case but you can get other brands that are very good as well.
You will most likely get the blue top but pay close attention to what color the case is. Light grey is a deep cycle and dark grey is cranking(dual purpose). Do not get a deep cycle for a starting battery. The Volvo manual even states not to do this as there isn't enough amperage to run the computer when cranking and can fry the board.
As for the charger, you should be able to use the existing on board charger. It may not be optimized for AGM batteries but you can use it on Optimas.
An example is I have a red top in my truck which I don't use every day but I do use once a week or every other week and I never use a maintenance charger. It just chargers off the alternator. Its the same battery except has different connections for a car so you can rapid charge with the same voltages as lead acid.
You don't need to leave the maintenance charger on all the time on an AGM unless you have something running. They don't have the same drain.
I know I was long winded and its up to you if you want to spend like twice the price and you may not get twice the years out of it but there are other advantages. I've done lots of research on batteries because I get bored in the winter... LOL! They work for me...
btw, a 800 amp lead acid is like 65lbs a 800 amp optima is 38lbs. that was the other reason I bought them, it help my list to port.
http://www.rodauthority.com/tech-stories/ignition-electronics-efi/the-hows-and-whys-to-selecting-the-right-optima-battery/