Decent 3rd party battery for X201

[pointyhat]

I've got an X201 with a 6-cell that is on its way out. From Linux:

[ph@x201 ~]$ cat /proc/acpi/battery/BAT0/info
present: yes
design capacity: 62160 mWh
last full capacity: 30780 mWh
...
model number: 42T4837

Can anyone recommend a vendor for a reasonable non-Lenovo 6 or 9 cell pack I can get for this unit? I looked at Lenovo but they want £145 for a 6-cell and £172 for a 9 cell which is more than what I paid for this whole machine.

I did consider rebuilding the pack as 18650's are pretty easy to get hold of but the charge controller circuitry is a barrier on this as it'd need reprogramming or resetting somehow.