Last week I decided to take out my M2P for a spin.  I've got the original 3 batteries, which are now about 4 years old and I've been worrying about their overall health, even though they've not been used much.  They'd been charged a bit over a week earlier, and when I went to fly, the battery was at about 60% charge. I'd planned on a short flight, so that seemed good enough.After only a few minutes, I got a battery error message, then another, sating there was only enough battery for RTH.  The drone made it about half way back over an area of paddocks and woods, before it announced it was out of charge and descending.  Long story short: I got super lucky and found it had come down in a tiny patch of clearing (only about 5m x 5m, in an otherwise forested area.  
So, I definitely need new batteries.  Batteries aren't cheap and the Mavic 2 isn't a new machine.  I know that degradation of LiPo batteries charging capability declines steadily from their manufacture date, so I'm trying to find a source where I can be reasonably sure they haven't been sitting on the shelf for a long time.  Aside from buying through the DJI store, are there other sources people can recommend where the price is competitive and they're not likely to have been laying around?  Thanks in advance for any advice!

You left the batteries for more than a  week after charging, and then took them for a flight. You should never do that, the batteries have self-discharged, and the indication of remaining power is not correct.
Try this: Charge them fully, and go for a flight the same day. Fly until you have 10-15% remaining, then let them cool, and charge them fully again. Then fly until you have 50-60% left, now you can store them.
I think you will find that your batteries are better than you think.

ArcticPhoto Posted at 5-28 01:01
You left the batteries for more than a  week after charging, and then took them for a flight. You should never do that, the batteries have self-discharged, and the indication of remaining power is not correct.
Try this: Charge them fully, and go for a flight the same day. Fly until you have 10-15% remaining, then let them cool, and charge them fully again. Then fly until you have 50-60% left, now you can store them.
I think you will find that your batteries are better than you think.

Thanks, @ArcticPhoto.  Thanks for the feedback, which is greatly appreciated.  I've not been flying as routinely as I used to, but I'd thought that one of the aspects of the "intelligent flight" batteries, was that they automatically would do the winding down of the battery, if unused.  I'll reset the batteries and have another go, fully charging them before the next flight.  

Hi there. Thanks for reaching out, and we're sorry you experienced such issues. In addition, the Mavic 2 Intelligent Flight Battery is a 15.4 V, 3850 mAh battery with a smart charge/discharge functionality. It features an (active) auto-discharging function. (Active) Auto-discharging function (when the battery is placed by itself): The battery will be discharged to 96% one day later. If the battery level is 70% or more, the battery automatically (actively) enable the auto-discharging function when it is idle for two days (default value). (It takes about three to four days to discharge from full battery level to 60% of the battery level. It is normal to feel moderate heat being emitted from the battery during the discharging process.)
After your tests, if you still feel like the batteries are continue to degrade, you may check link to see the stores near you. Alternatively, we hope you can get the best recommendations from your co-users here. Thanks for your understanding.

GMG Posted at 5-28 17:06
Thanks, @ArcticPhoto.  Thanks for the feedback, which is greatly appreciated.  I've not been flying as routinely as I used to, but I'd thought that one of the aspects of the "intelligent flight" batteries, was that they automatically would do the winding down of the battery, if unused.  I'll reset the batteries and have another go, fully charging them before the next flight.

Yes, the batteries will automatically "wind down", or discharge, to around 60% if you leave them unused. They are supposed to do that because they should not be stored at full charge.

But you should never put those partially charged batteries in the drone and fly. Because of the storage the cells in the battery may have become unbalanced, and when the battery gets a heavy load it will lose power faster than you would expect. Always charge your batteries fully if they have been stored for more than 2-3 days.