The exhaust manifolds in the 2.5L Fiero are found in several other GM cars but the manifolds in the 2.8L Fiero are unique. Unfortunately the 2.8L manifolds are prone to cracking and leakage. Repair by welding or replacement with used exhaust manifolds are the only options with a 2.8L engine (other than aftermarket performance headers or exhaust pipe modifications). If you are willing to make customized exhaust piping, manifolds from many 2.8/3.1/3.4 engine will bolt to the Fiero heads. Note that the Fiero manifolds are stainless steel.

That said, there is one set of cast iron exhaust manifolds that will bolt to the 2.8L Fiero which requires only minor modification to the downstream exhaust piping. These are the manifolds from the 1989/90 Grand Prix Turbo (Turbo ONLY). The Fiero Y-pipe requires bending to match the manifolds and this can be done using a vice and a 2" pipe for additional leverage. A benefit of using the Grand Prix manifolds is reduced noise in the cockpit.

A company called AM Autoparts (am-autoparts.com) lists a new manifold for the Fiero 2.5L engine for about $100 (March 2010) with free shipping.

Exhaust Manifold Bolts, 2.8L

We'll say it up front: Replacement bolts for the 2.8L exhaust manifold are available as Dorman Part No. 03411. It is a set of 10 studs with locking nuts for Ford V8s. Typically available at auto parts stores - ask for them by number.

'84-87 2.5L
Pontiac 6000              '83-91  2.5L
Pontiac Astro             '85-90  2.5L
Olds Calais               '85-91  2.5L
Camaro                    '82-86  2.5L
Chevy Celebrity           '83-90  2.5L
Buick Century             '83-92  2.5L
Olds Ciera                '83-92  2.5L
Chevy Citation            '83-85  2.5L
Firebird                  '82-86  2.5L
Pontiac Grand Am          '85-91  2.5L
Olds Omega                '83-84  2.5L
Pontiac Phoenix           '83-84  2.5L
Pontiac Safari            '85-90  2.5L
Buick Skylark             '83-91  2.5L
Buick Somerset            '85-87  2.5L

We have read that the exhaust manifolds in the 1990-1993 S-10, S-15, and Blazer will fit the '87-88 Fiero 4 cylinder engine but we have not been able to confirm this.

'88 2.5L
Chevy Lumina Car          '90-92  2.5L

'85-88 2.8L, Front: This manifold is unique. No interchange available.
'85-88 2.8L, Rear: This manifold is unique. No interchange available.
Aftermarket Exhaust Parts:
Exhaust Kit (flange, gasket, bolts), 2.5L,'84-88       Help! 03126
Exhaust Manifold, 2.5L, '87                            Manifold Plus 674-101
Muffler, all years                                     Walker 40094
Left megaphone tip                                     Walker 35866
Right megaphone tip                                    Walker 35867
Exhaust hanger springs                                 Walker 35722, 10 reqd
Left exhaust tip hanger                                Fiero Store 60189 
Right exhaust tip hanger                               Fiero Store 64158 
Catalytic Converter                                    rodneydickman.com