Iron comes from Iron Ore via the Forge. Iron Ore can be found in Iron Nodes in the Stalactite layer and Lava layer. The ore has to be harvested with an Extractor, as the node itself cannot be pulled with Power Cells, it also cannot be placed. All Ores can be smelted into bars in a Forge for further processing.
It depends on the substance (water for example has 1000 grams to a liter, other substances have more grams or less grams to a liter depending on a property (called the density measured in grams / liter) of the substance.
The key to working a liter to milliliter problem (or vice versa) is to know the conversion factor. There are 1000 milliliters in each liter. Because this is a factor of 10, you don't actually have to break out the calculator to do this conversion. You can simply move the decimal point.