Ores are placed in veins using Custom Ore Generation in various distributions. These veins are very, very long and are not terribly common, although it varies by ore. They will take a lot of work prospecting in order to locate, but once found should provide enough ore for a decent sized industry that will persist for a long time, even on a server with mutliple people.
Each block of ore will need to be broken multiple times before it is exhausted and turns into air, each break will drop 1 or more chunks of raw ore. How many times it will need to be broken depends on its metadata value, representing the density of ore inside the block. For veins this is typically 2-6, but the motherload center can be as high as 12-16. At the right, you can see several of the metastates of iron ore, starting with the lowest amount left at the top-left, increasing by 2 towards the bottom right. All compatible ores will function this way, each mining action reducing meta by 1, except diamond, which drops by 3.
All mods that break blocks should cause the ore to exhibit this behavior. For example, IC2's mining laser is required to punch through the full meta-hardness of the block, droping ore nuggets and reducing the laser's power, before the block is completely removed.
Each ore has its own associated indicator plant, as well, shown to the left. More information about each plant can be found on the prospecting page, including links to a description on wikipedia or to a scientific resource detailing the plant's mineral indicators and a link to a photograph.
Additionally, there is another source of iron added, limonite, or bog iron, which is found near sources of water such as rivers and oceans, as well as in swamps and under jungles. Limonite is resource intensive to process into usable iron, but is easy to find.