Harmonic Balancer: The harmonic balancer for the V-6 has been discontinued by GM but part number 14085401 will fit if the timing marks from your old harmonic balancer are transferred carefully to the new one. Don't try to use the timing marks on the new balancer; they are 60 degrees off!
We have no information on harmonic balancers for the 4 cylinder Fiero other than the aftermarket version shown below. Since so many 4 cylinder engines were used on other cars the balancers are probably still available from GM. It is also reasonable to assume that if an engine will swap into a Fiero (see the "Engine" section), so will the harmonic balancer from that engine.
A harmonic balancer for the 2.8L V6 (or any engine) can be obtained from the Damper Doctor, http://damperdoctor.com/. Their dampers are rebuilt with new parts where needed and the timing mark correctly placed. Quality is reported to be excellent. The cost is around $71 plus shipping.
Some aftermarket balancers for the Fiero are shown below.
Harmonic Balancer, 2.5L, '87-88 Dorman 594-027
Harmonic Balancer, 2.8L, '85-87 Dorman 594-013
Harmonic Balancer, 2.8L, '88    Dorman 594-008
From Auto Zone for Fiero V6 (in 2002):
Pioneer DA-1731 (replaces OE# 14083394) 73.99
Pioneer DA-1733 (replaces OE# 14085401) 67.99
1 year warranty on them. Auto Zone only sells new balancers.