The U.S. military is currently issuing the
M855A1 round for its 5.56mm assault rifles, which is a steel penetrator tip on a copper core. Steel also sees use in armor-penetrating ammunition as a less-expensive alternative to tungsten. (A protective coating such as Teflon is sometimes used on AP ammo to protect the barrel from wear due to the harder material of the round.) AP ammo options in Shadowrun are a bit limited: APDS doesn't specify, but I suspect that they're supposed to be tungsten. Still, a steel-tipped semi-armor penetrating rounds should, if anything, be cheaper than APDS.
Additionally, I would expect anyone with the Armorer skill to be able to cast their own bullets in whatever metal they had handy, should they need un-alloyed iron for some reason.
The entry for "Silver bullets" in Arsenal includes this little note:
Since many beings inhabiting the Sixth World are allergic to
certain substances, a wide range of projectiles with allergic substances
and alloys are manufactured in small numbers, with silver
bullets being just the most well known. It is up to the gamemaster
to determine the availability, price, and other statistics for such
kinds of ammunition.