Realistically, you have 3 choices;
1) Buy it from the Scimitar shop in Al Kharid
2) Get it as a drop from an NPC that drops it
3) Buy it from the Grand Exchange
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addy scimitar
The adamant battle axe and adamant scimitar are.
Grand exchange
If your talking about where it is in Runescape. It is in the commons in within Ape Atoll. Look for the scimitar icon on your mini map.
You can find this out quickly in the game itself: click on the skills icon, and click the "Defense" skill - the relevant skill for wearing most armour. In the case of adamant, the answer (for the new RuneScape) is that you need Defense level 40.