I assume this is a semi automatic box of some sort, in my opinion most auto boxes have a relatively short life compared to a manual and are always much more expensive to repair. I can only suggest that you get a used one from a breakers, have a look on ebay.

And if you add in the hours of labour to strip down and rebuild a gearbox,fitting a new assembly is probably cheaper!Especially as the gearbox change will probably be done by an apprentice/trainee under supervision and a gearbox rebuild would require a highly skilled fitter and still need to be taken out and refitted.