You get small bolts and washers etc from some one like pro-bolt.com.
You just have to work out what you need.
What I would do is collect all the bolts you wana replace and measure them all for length and Di.
Then you know what you need to order..
Derailer cable pinch bolts, cable guide bolt, bottle cage bolts and front mech clamp bolts can all be alloy.
Light weight top cap and extralite expander plug.
And replace any steel bolts on your stem and any light or garmin mounts with ti
Over a whole bike you could have 50-80g but a big chunk of that is the expander plug and top cap set up.