I am not surprised. The japanese companies are difficult to get spare parts for because:
1) They don't stock ANYTHING
2) They hate selling individual parts. They want to sell stuff in a complete set, the way THEY intended it to be bought.
3) Their definition of "special color" is . . anything but the one or two colors we need is "standard"
Your only other alternative is to pray and get lucky on Yahoo Japan, i guess
Also - i forgot, the SLs aren't "supposed" to have center caps, which is why the adapters were created. Same for the TE37 Ultra Track Editions
EDIT:
Crap! i just checked Evasive and the adapters alone are $60 EACH?!
https://www.evasivemotorsports.com/m...SABEgL_APD_BwE