OK
My experience is that Segway batteries should last about 10 years, if well looked after.
Your batteries should NOT yet be approaching end of life;
C051414xxxxx
Manufactured May 14 2014
C091813xxxxx
Manufactured Sept 18 2013
As I recall, the screws are M4 ( metric ) cap screw, 30 mm
If you cannot find at your local store, there's always eBay like this;
https://www.ebay.com.au/itm/10xM3-M4...t/122705877050
( i have not used them, but hey for a couple of bucks how far wrong can you go )
Good Luck.