Didn't realize they don't have WalMart down under o_o Basically any store that has a pharmacy section should carry it. It's less habit forming than actual sleep aids.

Depending on how long you have to get your sleep pattern right depends on how heavily you'll want to keep active during the day and what dosage you'll want to take the melatonin at. As a note - a 10 to 12 year old child can take up to 50 mg in a night. Start at somewhere around 20 (for adult dosage), and if after 30 minutes you aren't falling asleep take another 10-15 mg. Don't go beyond 3 doses in a night. At that point you're wasting energy. ((And if it takes that, start the next night at a higher dose))

What's super important is - don't sleep during the day if you can help it. That's what focusing on being active is for. The idea is every day you push your body forward should make it that much more likely that you'll fall asleep when you go to bed the next night.
DO NOT DRIVE WHEN PUSHING YOUR BODY LIKE THIS, YOU'RE ASKING FOR AN ACCIDENT IF YOU DO

If you can get your body to go three nights sleeping at night and awake during the day, you can count it as beat. IF after that you find your sleeping pattern relapsing, talk to your dr about needing a sleep study and considering medication to regulate your serotonin/melatonin levels. Chances are you'd be dealing with a chemical imbalance at that point.