I think any 12V 7Ah battery would suffice. Just make sure when connecting you get the polarity right. Check with a meter to ensure centre of plug is + or - depending on what it needs to be since you can reverse it by connecting the wires opposite.
See also
https://www.nexstarsite.com/OddsNEnds/PowerSources.htm
Just search for sealed lead acid batteries (SLA) often incorrectly called gel cells. Amazon sell them too for a bit cheaper. You can get 9Ah for less than cost of 7Ah at elsewhere but there’s not a lot in it.
Edit.
The 12V 7Ah or 9Ah batteries are fairly small and light and can sit on your tray between the tripod legs if you have one or on the ground. Get yourself a multistage 1-2A charger and when you’re finished with the battery for the night just put it back on the charge. This is what I am doing with my 12V 9Ah SLA which also happens to be deep cycle (it was cheap <$30 and has served me well this year). The charger may set you back about $90 for a 1.5A model which is all you need for 7-9Ah.