It's hidden in a small clearing in the middle of the forest. It's a little tricky to find, but here's how you can get it:
1. Enter Ilex Forest: You can reach the forest by following the path from Goldenrod City.
2. Head Towards the Center: There's a main path that goes through the forest. Follow this path until you reach a clearing with some tall grass and a few trees.
3. Search the Clearing: The Metal Coat is hidden in the grass in this clearing. It's easy to miss, so look carefully. You'll need to use the "A" button to search the grass.
Once you find the Metal Coat, you can use it to evolve Onix into Steelix.
Tip: You can find a lot of other helpful items while exploring Ilex Forest, so it's worth taking your time and looking around!