Hi my_kill -
It looks like all three buttons are a single part, number 4-678-917-02.
The exploded parts diagram gives the part the description “Button L” .. but it appears to be the plastic for all three buttons.
The part is not currently in stock at Sony Direct Accessories & Parts Center. It’s priced at US$47.38, plus shipping.
Nexttronics also lists the part on their website. Also listed as out of stock. My experience has been that Nexttronics is able to obtain out of stock items faster than Sony DAPC (or maybe their order fulfillment is faster and lead time is the same). Nonetheless, the price is the same.
Jef