How to make/get obsidian in minecraft pocket edition without lava?

I want to make a nether portal but I have no lava to make obsidian. Is there a way to get some or make some without lava involved? Like mining it? Or using flint and steel?

You have to get the nether reactor to go to the nether.
You need
- 4 blocks of cobblestone (easy to get)
- a nether core (made with diamonds and cobblestone)
- 4 blocks of gold (hard to get 36 gold ingots but you will gat them eventually)