Zgrt does c and d blanks, Luftverk does c blanks,
various markers 3d print blanks. Myself, @AudreySickburn and a few others I’m forgetting
Soul fuel makes teak and brass blanks
https://www.instagram.com/soulfuelskilltoys?igsh=ZDcxbGxrMHpxazhp
@Xanadu makes epoxy filled blanks
@Saltman360 has tinkered with blanks for his RTD /Rover designs that fits C/D stem kits.
Lathed back designs makes blanks
@Fellhoelter makes c blanks and maybe other blanks if we nudge hard enough.
@Lotaxi has a lathe and might make a ti blank if we beg 
Mcmastercarr sells spools/blanks and bearings of various standard sizes
I’m probably forgetting folks but lots of blank options depending on size/material your looking for