I believe it's possible to make truly unlimited obsidian with Nether portals. As a Portal will be created in the Nether if one does not already exist, you could do this:
Gather enough material to make two portals (20-28 Obsidian, 1 Flint, 1 Iron Ingot)
Construct Portal A in the Overworld
Travel to Nether
Destroy Portal A in the Nether
Move ~32 blocks away in a known direction (East, for example)
Construct Portal B in the Nether
Travel to Overworld
Destroy Portal B in the Overworld
Profit: 28 Obsidian
Without any further resource expenditure, you can travel through Portal A in the Overworld to generate 14 more Obsidian blocks through a new portal, and do the same for Portal B.
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Spontaneous combustion - one of the many hazards of travel between worlds.
Another way to get obsidian is to go to the End (it helps to defeat the dragon first). First, you create a platform directly underneath the obsidian platform you spawn on. This may be tricky depending on where you spawn. Then mine the obsidian platform (25 blocks) and go back to the main world. The next time you go to the End, a new obsidian platform will spawn in its place.