First, on the availability of iron:

As a first approximation, iron ore is everywhere. The quantity and quality do matter, since with more abundant and richer ores, you can get enough ore for smelting with less labour (thus, cheaper iron), and some ores produce better iron than others. Good deposits of ore can allow a large-scale centralised iron industry, with more concentrated expertise, larger smelting furnaces (which can give better yields of steel), and better transport networks. But without good deposits of iron, bog iron will often be available, allowing iron to be made (but usually in small smelters dispersed over a large area).

Obtaining iron ore was discussed in https://www.reddit.com/r/AskHistorians/comments/9c1ezj/medieval_mining/ with u/mikedash providing a useful array of links to earlier discussion, and u/Moffiker giving a detailed reply. Note that even when the source of iron is iron sand or bog iron, the collection of the ore might only be a small part of the cost of the iron. For example, I discussed an Indian example in https://www.reddit.com/r/AskHistorians/comments/8y6u5g/how_iron_mining_differed_from_bog_iron_harvesting/ where the ore was only 20% of the cost, with the other 80% being equally divided between the charcoal and the actual smelting. (Note that for pre-modern iron, charcoal is often the limiting resource.) Ore sources such as bog iron are too dispersed to sustain a modern industrial iron industry, but they were often thoroughly sufficient before modern times.

As Pliny the Elder wrote in The Natural History,

Deposits of iron are found almost everywhere, and they are formed even now in the Italian island of Elba, and there is very little difficulty in recognizing them as they are indicated by the actual colour of the earth. ... Iron ore is found in the greatest abundance of all metals.

Pliny's comment that

In Cappadocia alone it is merely a question whether the presence of iron is to be credited to water or to earth, as that region supplies iron from the furnaces when the earth has been flooded by the river Cerasus but not otherwise.

suggests that the Romans knew and used bog iron (as one would expect).

Second, on the necessity of iron:

You asked:

So where did the Romans got their iron to fight successfully against the Etruscans?

In principle, it isn't necessary to have iron in order to fight successfully against people who have iron. Iron can be very useful, due to the above-noted abundance of iron ore. Compared to bronze, which is often expensive due to scarcity of tin, iron is cheap (at least, once an iron industry has been established). Cheap iron can allow a larger army to be equipped than expensive bronze. While iron itself might not be superior in performance to bronze (its hardness is similar to bronze, and depending on the quality of the iron, bronze can be tougher), its cheapness can make it the sensible military choice. (While iron isn't mechanically superior to bronze, steel - especially quench-hardened steel - can be enormously better for making weapons (and also enormously better than iron (i.e., steel with too low a carbon content to be hardenable).)

Third, on the use of iron:

The typical Italian warrior/soldier of the time - Etruscan, Roman, and others - would have worn bronze armour, if metal armour was worn (including a bronze helmet), and a shield largely made of wood (which might have a bronze facing). Spears - very important weapons in battle - might have iron heads or bronze heads. In the earlier wars between Romans and Etruscans, bronze swords were still in use (e.g., double-edged antenna swords), and bronze battle-axes, although the iron-bladed kopis was around (from about the 7th century). The iron-bladed kopis (and makhaira) became more common, largely replacing bronze swords and axes by about the 4th century (double-edged iron swords were also in use, but appear to have been less common than the kopis/makhaira).

Being metal-poor would have been a bigger problem as far as bronze went, since the typical well-equipped warrior carried more bronze than iron. Iron, even if the Roman iron industry was smaller than the Etruscan iron industry, would have made things more even, if lack of bronze was a problem (and if lack of bronze wasn't a problem, then lack of iron wouldn't have been a problem).