Any source for this hose?

According to the factory books (using several sources) It goes from the air fitting at the cold start valve to the auxiliary air valve. That would be functionally correct and the hose pictured is the same unusual shape as yours. It is no longer available.
Does your car even have an auxiliary air valve?
In similar situations I have had to make hoses.

 

I think the SPC does (confusingly) show that 115226 is the right hose for your car (even though you can't buy one ). In the euro 308QV SPC, they show two different configurations sort of mixed together -- one where the AAV is mounted on top of the engine, and the other where the AAV is mounted under the coolant expansion tank. If you look in the SPC figure:

https://www.ferrariparts.co.uk/diagram/ferrari/308-quattrovalvole/010-fuel-injection-system-fuel-distributors--lines

you can see the metal elbow, item 73, in the upper left figure detail, and the notes say that this was introduced at 44159 (GS/LHD) and 43961 (GD/RHD), and this would use the AAV, item 30, shown mounted under the coolant expansion tank; whereas, lower SN chassis would have the AAV, item 30, shown mounted on top of the engine (and use the tube, item 34). Since the hose connected to the metal elbow 73 going to the cold start injector fitting is shown in phantom and does not have a different item number nor a different part number = it's still the 115226 hose.

Ferrari does this in many places in the SPCs where not everything shown in every SPC figure is used on every car = it's terrible!