As a last resort, I think you can mod the game files to make other more common stone types act as metal ores, but I've never done it myself and don't know how.
It's actually REALLY easy to do this. I do it for Cobaltite (normally an ore of non-existent cobalt) to make sure I always have iron at every site. All you need to do is open up the inorganic_stone_mineral.txt file in the RAW folder (Dwarf Fortress 0.31.25\raw\objects). Inside will be a collection of stones, each one named clearly and all of them followed by lots and lots of values in square brackets (called "tags").
Add this tag to the stone you want to become an iron ore:
[METAL_ORE:IRON:XX]
Where XX is a value between 01 and 100. This represents the chance of success (getting an iron bar) vs. failure (and getting nothing). Setting this to 100 for something like Cobaltite means that Cobaltite is no different than Limonite or Hematite, and every smelting will produce an iron bar. Just paste it in below the stone name you want to change (but above the start of the next stone type) and the stone will become an ore of iron. As a balance consideration, if you change a stone that appears in layers (like, say, Marble), or is very commonly found in large clusters (like, say, microcline), you may wish to consider setting the success rate low, lest you drown in iron bars.
Now, generate a new world, and the change you have made will be a part of the new world you create, and Cobaltite (or your choice of stone) will be an iron ore. Once you get the hang of how tags work, it's a short trip from there to doing things like making all soils valid as both sand and clay, or removing aquifers or making all animals non-exotic (and thus trainable). Saving kobolds from starvation extinction with [NO_EAT] or allowing sun berries to appear in this version by removing the broken [GOOD] tag, and so forth.
As a more concrete example, here's the entry for Cobaltite in my RAWS, with the added tag bolded:
[INORGANIC:COBALTITE]
[USE_MATERIAL_TEMPLATE:STONE_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:cobaltite][DISPLAY_COLOR:1:7:1][TILE:158]
[ENVIRONMENT:IGNEOUS_ALL:VEIN:100]
[ENVIRONMENT:METAMORPHIC:VEIN:100]
[SOLID_DENSITY:6295]
[METAL_ORE:IRON:100]
[IS_STONE]