Assuming your watch has a Cal. 501, and assuming the part in question is the barrel bridge (there is no part called a mainspring bridge), it is still available from Omega. Part number is 72205001001 but it's not cheap. I would also ask them why this bridge needs replacing - if the hole for the barrel arbor is worn for example this can be repaired with a bushing.
You are fortunate it's still available, as not many parts are for this series really. Although finding donor watches for parts is not a bad idea, be aware that there is a chance that if the part you need is a high wear item, it's likely going to be worn in the donor watch as well.
Cheers, Al
