In terms of melatonin, I don't think there is any clinical proof of any long term side effects. It is simply a boost to your body's natural sleeping system, and does not suffer from the side effects of other prescription sleeping pills. You can buy it like paracetamol in almost any country in the world, it is only really in the UK where they feel the need to put it on prescription (for financial rather than health reasons I feel). In terms of getting it, my dad spends a lot of time working in Hong Kong and the US so he picks up the bottles when he's there. I suppose if you could ask your doctor for a prescription trial and then if it works and you know anyone traveling they could get some for you?
I have also used 10mg zolpidem and found them to be completely useless after one night. On the other hand, I then switched to something else - really sorry but can't remember the name - and 3.75mg of that works brilliantly. So I guess if something isn't working for you, then I would recommend going back to your GP and asking to try a different medication. Hopefully you've got a decent GP whose not automatically averse to prescribing, it is their job after all!
Good Luck xx