Good morning, I hope I can help you.
There are specialist shops that will rebuild/repair your gearbox but for a replacement you are best sticking with BMW original equipment.
Bill.
Where about are you and what is the fault with your gearbox.
Try contacting Hardy Engineering on 01372 378927 and discuss with them your problems and requirements and if they will give you a quote.
You can also try this number 01323 846666 and compare prices with above.
UK Car Mechanic
City & Guilds