Unfortunately, it's unlikely you'll find anything worthwhile at this point. Phone batteries are specific to each model, so LG would've stopped production of them not long after the phone ended its run. Third party manufacturers may go longer, but you never know what you'll get. Even they wouldn't produce batteries for a largely forgotten phone with a troubled history (the infamous boot loop debacle) this long. At this point, anything you buy would be a crap shoot. Even if you bought a well designed battery from a reputable seller, it'll be old stock and already degrading.
Unless you are willing to replace the phone, I only see 3 options, but each with tradeoffs.
-Buy several standard batteries. You may get lucky and one will still be well serviceable, but you're more likely to just swap them all out during the day. Additionally, buy a stand alone charger to charge them when not in use.
-Buy a larger battery. Even if it has reduced capacity, it may still be enough to get through the day. The downside is it makes the phone bulky and it still may not get you through the day depending on how bad it is. I.e. It may have the capacity you need, but enough internal resistance build up that you get excess voltage sag that shuts the phone down with a heavy workload.
-Buy a case with a built in battery, or external battery pack. These may use more standardized batteries that get you a fresher one (though the case batteries are still risky due to being an old design), but you then either get to deal with a bulky phone or tethered to a battery pack.
Personally, I wouldn't do either. As old as that phone is, it's just time to upgrade for multiple reasons. That would be my preferred route. If that's simply not possible, then I would just leave it as is and keep chargers and cables accessible to charge as needed (or go the large battery pack route if access to an outlet isn't possible).