SkiffRopes sell Hales Micro Radio Control Yacht blocks. The blocks have a 7.3mm sheave, ceramic ball bearings, M2 Stainless Steel fasteners, 16mm between mounting bolts and are light weight (starting at 2g). The ceramic ball are insulators, meaning there is no metal to metal contact anywhere in the moving parts. This removes the possibility of electrolytic corrosion.
See supplier info at: http://www.halesmicro.com.au/index.html
SBS-1c Single Block with Shackle
DMB-1c Deck Mount Block, Single Pulley
DMB-2c Deck Mount Block, Double Pulley
BBS-2c Double Block with Shackle
SB-1c Single Block with Screw
SMB-1c Surface Mount Block