1. 924 mid 82 and older - stamped steel - VW/Audi part# 477.407.153F (left & right)
2. 924 mid 82 and later, 924S, 944 83-85.1 - stamped steel - VW/Audi part# 171.407.153D (or for track) (left & right)
3. 944 85.5-86 na/Turbo - Cast aluminum - 951.341.027.00 (left), 951.341.028.00 (right)
4. 944/968 87-95 na/S/S2/Turbo non-M758/M030 - cast aluminum - 944.341.027.02 (left), 944.341.028.02 (right)
5. 944/968 87-95 M758/M030 & 89 Turbo - Cast Aluminum (larger slotted pin) - 951.341.027.31 (left), 951.341.028.31 (right) [PCNA superceded part# - change 027/028 to 927/928]
6. A modified M030 style arm is reported to be available from Porsche Motorsport North America and carries the part numbers 951.341.027.32 and 951.341.028.32.  The newer arm is recommended for use in endurance competition.  Good luck finding them:-)

Discussion:

1. Steel arms are very cheap, and, can be retrofitted to 85.5-86 944's. The advantage being the affordability and ease of ball-joint replacement. These steel arms can be reinforced by box-welding the seams for use in 83-86 944 race applications.
2. Rebuilt Aluminum arms can be found for as little as $150, though these are NOT recommended for use in extended track applications.
3. New OE Porsche aluminum arms for non-M758/M030 cars can be found at dealers and select others for ~$450-600
4. New OE Porsche aluminum arms for M758/M030 cars can be found at dealers and select others for ~$750
5. Aftermarket Race application A-arms (Racers Edge) go for ~$1600/set. These billet aluminum pieces feature stronger overall construction, replaceable/serviceable ball-joints, spherical cross-member bushing, and spherical caster eccentric bushing (optional). They are direct bolt-on for 85.5-86 and 87-95 944/968 cars, and could easily be applied to 83-85.1 944's as well.  Paragon Productscarries these arms and caster eccentrics.