You can buy one from the grand exchange or their are some hatchet shops around runescape.
All adamant armour like Adamant full helm, Adamant platebody, Adamant plateleg
Grand Exchange
there is not another place you can buy a addy halberd besides the ge
The adamant battle axe and adamant scimitar are.
There are only a few. The best drop is the adamant med helm so this is not recommended. Attacking Cockroach Soldiers are better as they drop Rune Scimitars and Sq. Shields. Note: It may take about 15 kills to get a good item. Most of the time the drops are coins.
The level 83 cockroach soldiers drop them on occasions in runescape, for nonmembers and members.
Please use the Runescape forums for things like these, that is what they are there for.
Adamant Dragons do not exist. The highest-tier metal dragon is still Mithril.
Darts and throwing knives are the fastest range weopons. Scimitars are the fastest malee.
Get about 5 rune scimitars from the highest bestiary in the Stronghold of Player Safety.
The easiest way to get it is to buy it from the Grand Exchange. There is also a plateleg shop from which you can buy it, but it's actually more expensive there. Yet another way to get adamant platelegs is to smith them yourself, from adamant bars - but this, too, is more expensive (due to the price of the bars) than actually buying the platelegs on the Grand Exchange.