hi and welcome,you should be able to pick one up between
500-1000 depending on condition,to change it yourself is a ball ache if you don't have a ramp.there are traders on here that can sell you a secondhand box and fit it for you.
What symptons are you having with the gearbox? May be worth considering a gearbox rebuild rather than buying another unknown second hand gearbox. Price for a rebuild will depend on parts needed of course.
As said without a ramp it is a fair job!
I have a couple of 2 poster ramps gearbox jack ect ect. I have just not done the job before and was hoping there a step by step out there.
As for gearbox problems. When u bought the car it was driving fine but once there was heat in it or boosting it was very stuff to get in gear and was a real pain.
Then today I went from 4-3 and bang very large clunking noise if you stick your foot on the clutch it go's away. I can still select all gears but when i realise the clutch pedal the clunking starts again also no drive.