"seat buns" you mean the foam deals which fit inside the coils? That's a MB dealer part, I think it begins with 126- ... I know Whunter had a post on them somewhere.

If you can find a nice JY passenger seat bottom, and more specifically the spring box, you got it made. Take that, then cannibalize another seat spring (now you have double springs), and I highly doubt you will need the foam inserts. I did this to both of my seats and they feel great, and probably better than new. Re-use the metal clips which hold the springs together for that factory finish. I have photos somewhere in a thread /post on this topic. I used to sell re-built/mod'd seat bottoms but the JYs rarely have older MBs now ...