I used IRF520/9520 for a short time in my prototype and didn't hear an appreciable difference once I swapped to IRFP140/9140, though I also wasn't listening for that and didn't do an A/B comparison. I think there is value there if you're interested in finding the subtle differences.
eBay is an excellent place to buy shoulder washers, silicone insulators, and bolt-nut-splitwasher-starwasher combinations. If you are willing to search and evaluate for yourself. If unwilling, Mouser has all of the requisite pieces but you still need to search for yourself.
Going by the listed specs, I only found a single insulating shoulder washer on DigiKey that will fit a TO-220. As Mark says, eBay and Ali are reasonable sources for that type of component, but you do have to check dimensions carefully. I've got more than one bag of shoulder washers for TO-220s where the length below the shoulder is greater than the typical thickness of the transistor tab, so they don't actually clamp the transistor to the heatsink.
There are a few different styles of clamping bars that bypass the need for shoulder washers by pressing on the body of the transistor. I've been meaning to buy a variety to try out, though maybe not on this particular amp / chassis.
There are a few different styles of clamping bars that bypass the need for shoulder washers by pressing on the body of the transistor. I've been meaning to buy a variety to try out, though maybe not on this particular amp / chassis.