Its difficult to point you to the right direction without knowing what type of bike you have interest in restoring.

But for starters, join online groups of the same bike so you can leverage the knowledge base. Bikes will undoubtedly come up for sale there also.

working on an old bike is not an easy task nor a cheap one and sourcing parts will be immediate challenge. It depends on how well you want the restoration to go, or how "original" you want it back. Again, you would want to lean on available resources from the group.