" I hear a hissing air sound when I started it and it raised up slowly in drivable mode"
This should make it very easy for someone to find the problem, just a spray bottle with soapy water, it could be something as simple as a line repair.

 

If you need a bag, you can also get Arnott bags from advanced auto:
Buy Arnott Air Suspension Air Spring A-2202 at Advance Auto Parts

Some coupon codes:
TRT41 - $40 off $100
ES123 - $20 off $50
A124 - $10 off $30, $25 off $70, $40 off $110
HUGE50 - $50 off $175

You can get it as low as $108. (orders over $75 have free shipping)

I don't think it is an airline issue.

From: Disco 2 Disco II Air suspension – the definitive guide! - Land Rover Zone

Sinking on one side or the other is a sure sign of an air leak on that side, if it regains normal height after running the engine (remember doors closed!) then the compressor is ok and you should be looking at replacement air bags. Replace them in pairs!
Spraying soapy water on them when in ‘extended height’ mode is a good telltale for an air leak. Find the bubbles and you have found the leak!
Expect to pay around £90 each for Air Bags and £0.50p for the clips – you’ll need two clips for each air bag. The clips are reusable, but may break when getting them off. Get some anyway to be sure.

Very few people have had an air pipe wear through by rubbing on the chassis, this is not a common fault. If this happens then the compressor will be constantly on (when the engine is running) and the car will not lift on that side.