* Route 35: This is the most common place to encounter Onix.
* Route 46: Another good spot to find them.
* Ilex Forest: You can find them here as well, but they are rarer.
To evolve Onix into Steelix, you need to trade it while holding a Metal Coat.
Here's how to get a Metal Coat in SoulSilver:
* Buy from the Department Store in Goldenrod City: This is the easiest way to get one.
* Find it in the wild: It's a rare item that can be found on the ground in certain areas, like the Ilex Forest.
* Get it from a Trainer: There's a chance some trainers in the game might carry it, but this is even rarer than finding it in the wild.
Once you have a Metal Coat, give it to your Onix and trade it to a friend or use the GTS to find a willing trader. The trade will trigger the evolution and you'll receive a Steelix in return.