Suspect my gearbox has gone kaput after 43000 miles

30 track days and 50% extra torque probably didn't help.
I now have zero drive in 5th or 6th gear, the revs just go up as if the car wasn't in gear and absolutely zero impact on drive.
4th gear down are functioning, but some funny sounds when changing into gear so pretty sure something is broken. I haven't driven the car since it limped today.
I don't think it is the clutch, at least a pretty mechanical minded friend who was in the car with me when it first happened didn't think so, mind you would be handy if thats all it was.I was driving nornally at the time and accelerating at low revs in 5th
Anyway, any suggestions as to the cheapest way of getting a replacement engine?
i.e. how easy is it to get what is presumably a pretty rare gearbox from a breakers yard ? Any links / advice etc. appreciated?
In general does anybody know who feasible it is to rebuild a gearbox and what sort of money effort would be involved?
Wonder how much a new gearbox is , supect its not cheap!
Unfotunately I will need to get the existing LSD added to the replacement gearbox which will add to the required labour (some machining required I believe).
If the gearbox is rebuilt would the LSD related labour be less than installing it in a new gearbox?
Seeing as I am on sabbatical at the moment this was the type of unanticipated expense that I could have done without.
No-brand cornflakes for me for the next 8 months
Cheers,
TB