You can get 10W60 oil lots of places, but if you want the Moto Guzzi specified oil, 10W-60 API SG, only eni (formerly AGIP) makes it, as far as I know. You can get it on ebay or at AF1 Racing.
I won't repeat here why Moto Guzzi specifies API SG instead of the current SN grade, but I do believe it is important if you want your tappets to last.