How to replace my MacBook Pro battery?

I have a 15 inch, Mid 2009 MacBook Pro, whose battery is kaputt. I live in the UK.

I accidentally switched my charger with a 13 inch one and the battery is now very unsteady. It only has about 1 hour of battery every time I unplug it, but it runs fine while plugged to the wall.

I wanted to know if you could help me finding a replacement battery as I have searched on the internet but am not entirely sure which battery model I should buy…

Ideally I want to replace it myself, because spending £100 in an official Apple store is quite a lot… However, if I don’t find anything online I will have it replaced by an Apple Genius.

Here are some other stats that may help you to help me :)

I ran this on the Terminal app _ioreg -w0 -l | grep Capacity

and this was the outcome:

“MaxCapacity” = 3924
“CurrentCapacity” = 3924

“LegacyBatteryInfo”=
“Amperage”=285,
“Flags”=5,
“Capacity”=3924,
“Current”=3924,
“Voltage”=12382,
“Cycle Count”=867
“DesignCapacity” = 6500

Thank you!