You'd get them straight from Yamaha (and it has to be the *same* yamaha as your board; e.g if your psrs910 is from Yamaha USA, you'd have to contact Yamaha USA. Assuming they still had them; any generic (shared across the line) parts they'll still have, but generally speaking most manufacturers keep product *specific* parts for only about 5 years (but may have old stuck up to 10 years past). So for example, if your screen went, it's the same screen as PSRs950, which launched in 2013 and was discontinued in 2015, so they would've kept stock for that screen up until 2020-2021, but very likely would have a couple of leftovers still now). On the other hand, the silver casing for the s910 (launched 2008, discontinued 2012) it's unlikely Yamaha had any casings left past 2018.
Mark