I have tried a few different replacement ear pads for the HP-1 and YH-1 Yamaha's. Per Joop Nijenhuis' HP-1 refurbishment pages I used Sony ear pads for my first HP-1. Those were Sony 211566702 ear pads. They work pretty good but seemed kind of pricey for what you get. The dimensions of the HP-1 ear pads are 7cm (outer diameter) and around 3cm (inner diameter - the opening). The Sony 211566702 ear pads have a lip that makes the outer diameter 7cm but the real diameter is 6.5cm.
I still have the original ear pads on my YH-1 headphones and other HP-1 headphones but I plan to replace them at some point. In preparing for that I have found some other ear pads that have the same dimensions (or close to it). I looked at the Yamaha RH5M and RH5MA headphones and saw that those pads fit. That part number is Yamaha CX810511 and is the 7cm outer / 3cm inner. They are pretty much like the Sony 211566702 ear pads. I ordered them from the Yamaha parts store.
I also bought a pair of Sennheiser 075527 ear pads (from Skygeek). Those ear pads are for Sennheiser HME 25 and HMEC 25 headphones (used in aircraft it appears). They are also 7cm/3cm plus they also have the built in protective screen for the driver. They are pretty nice
Next, I tried these less expensive Sony MDR-V150, V250, V300 substitute off ebay. They had correct dimensions but feel the cheapest. They might do in a pinch but I don't plan on using them and stuck them in a box somewhere.
Last, I tried these 7cm ear pads with protective screen from Amazon. I think they are nicest of the bunch and will be my choice for the next Yamaha HP-1/YH-1 refurbishment.