Unfortunately, I haven’t seen any below 84Ah. And as they are flooded batteries, portability isn’t a strong point.Re the battery, I've attached the datasheet for the one I intend to buy. 10kg and costs €120, and rated for 300 cycles at 100% depth of discharge. I was rounding-up when I said 40Ah, it's actually 33Ah @ C10 and 36Ah @ C20.
Do you know what a tubular-plate 40Ah battery would cost (and whether it's bigger and heavier?). This application needs to be portable, it's primarily for acting like a mini indoor generator for running low-power essentials during power cuts (computers, modem, lights and stand fans).