Cramp and Salt

For what it's worth - Time was I couldn't run for more than two and a half hours without getting severe cramp in the inner quads. Very painful but funny for spectators to watch me in my 'death throes'. By taking salt tablets I was able to extend these runs to three and a half hours. Then I discovered that by taking a Quinine Sulphate tablet, on prescription, I was able to get may be 6 or 7 hours of hard hill-time, if the weather was crap. Stopped taking salt then. However experiences since, suggest I should continue to take salt as well, and drink more than I do. Will definitely try taking dioralyte during a race and may be some savoury food beforehand.
Salt tablets seem difficult to get now - any ideas ?