The Alfa 6C 2500 Suspension is an original (Vittorio Jano) design dating from the mid 1930s and almost all of the parts were manufactured by the Alfa Romeo factory. The exceptions are the conventional ball bearings in the front hubs, rear axle and king pin thrust bearing which were supplied by RIV. There are a number of rubber bushings in both the front and rear suspension which can be reconditioned with new rubber. There is a famous supplier of 6C 2300 and 2500 parts in Milan called AFRA sas. They have a website. Much of their stock of original parts is now exhausted however they have re-made a lot of parts over the years but stocks tend to come and go.
The front shock absorbers are unique Alfa units as they combine both the springs and the shock absorber in the single unit. They can be serviced, adjusted and refilled with fresh hydraulic oil. They have a complex seal in them but it can be serviced with after-market parts. The pull rods in them suffer corrosion and wear.
The rear shock absorbers varied from model to model. Some had two per side and some had one per side. I am familiar with the one per side units and they are very large. They were developed by Alfa Romeo and manufactured in the factory or supplied by RIV at a time when telescopic shocks were rare on cars. They have complex seals but they can be serviced if they are not badly corroded.
The hardest part is the front suspension repair. The trailing arm and king-pin bearings are Alfa manufacture and are pretty well unique to these and the 8C cars. All of the bearings use 3 mm X 16 mm or 20 mm needle rollers. The inner shafts or shells and outer shells of each type of bearing are high precision Alfa manufactured parts with dimensions unlike almost any other manufactured roller bearings. They are made from forged material to fit the specific Alfa design and to seal out the road dust and enable the bearing to be greased in service. The steering box uses similar type roller bearings.
The only practical way to restore these is to find or make the correct parts or replace them all with custom made bushes in place of the needle rollers. Even doing that will require a considerable amount of precision machining. Substituting the originals with proprietary bearing shells is not a practical option. The original bearing shells tend to suffer brinelling and become “grooved” and un-serviceable. If repaired using bushes, the car will not perform with anything like the same feel as an original low friction needle roller system.
AFRA have made sets of these parts in the past but I cannot vouch for the quality and they no longer seem to stock them. They do currently stock reproduction ball joint bushes for the tie-rods. Do not be fooled by any one who tells you they can repair your suspension with cheap off the self parts.
There may be some outfit who have made reproduction parts recently.
They will not be cheap.