Buying new sockets will be a hell of a lot easier if any are available, but I've fixed an intermittent output socket contact with brute force:
3mm female bullet crimp:
https://www.amazon.co.uk/dp/B00UONOFHM
1. Turn off and open battery pack, cut off heatshrink, and unsolder the two wires from the output socket
2. Warm socket with hot air gun and dig out the grey plastic from around the six socket solder pins
3. Cut/lever apart the two plastic halves of the socket (warm again first)
4. Heat a little with soldering iron and pull out the loose socket insert (tell which one is loose using 3mm copper wire)
5. With drill bit held in pliers, widen hole with 4.0, 4.5 and then 5.0 drill bits (but dont drill through the front face plastic)
6. Grind off 1mm from front of new crip with Dremel so socket contact pressure point is closer to the front (otherwise probably won't make a good connection onto the controller pin)
7. Insert crimp into hole
8. Squeeze together the wire end of the crimp to close the open gap (to later solder into)
9. Apply superglue and hold two plastic halves together with needle pliers
10. Unroll the front of a second socket crimp and cut off to create an approx 10x10mm plate
11. Cut small middle pins as short as possible to avoid short circuits to the plate
12. Glue plate to plastic sides to restrain the new crimp socket from pushing out
13. Resolder wires (+ and - are marked on the front of the socket plastic, the + side locates on the charger input side of the battery pack)
14. Use Dremel grinding wheel to remove a little plastic on the case to allow the connector to refit with the added plate