I've had the best luck locally finding it at Rite Aid, although if you have health food stores, they'll almost certainly have it. The fizzing calmag was recommended by a friend with long experience in insomnia - the problem with a lot of sleep aids is that they only treat falling asleep, or staying asleep. For people with complex insomnia (which from the bits you've said, sounds like you have; both a problem falling asleep and staying asleep), it's necessary to find something that knocks you out, and then something else that keeps you out. For me, calmag + camomile is the right combination (well, at least sometimes - other times I'm just screwed, and take the time to listen to Stephen Fry read me Harry Potter, or somesuch).

Anyhow, the whole point before I went off on my own sleep deprived semi tangent, is that you want to make sure you find the fizzing powdered stuff. Because of its form, it's absorbed immediately into your system, as opposed to pill form, which obviously takes longer. You'll want to make sure you mix it with very hot water (so it all dissolves), and then mask the bitter taste with something citrus-y. (I like tangerine OJ for it, but mileage varies.)

Luck! Chronic insomnia is one of the most draining things.