Leaking transmission in Falcon

The transmission in our AU Falcon, which has done 490,000km, disengages when shifting from first to second gear when it is cold. I think it does it because of a lack of transmission fluid as the gearbox leaks a bit. Bearing in mind that the car is hardly worth spending any money on, do you have any ideas on how I could fix the leak cheaply so that we are not just pouring oil through it?