You may be able to find certain parts at Yamaha.com. They do sell to anyone, but parts may be limited to just common parts, like keytops, bass strings, etc. Any piano tech should be able to get you a replacement bass string, but would need to send in the broken string to the factory for exact match.
Most Yamaha pianos and eps have used the same keytops for many years. I just had to order an octave for a client that had 2 keys damaged by "piano movers". The octave was around $15 plus shipping.