Garhauer and Rig-rite would be my guess
If your O35 is like mine, most of the blocks, organizers, etc. are Garhauer, and the mast and boom are Isomat. Garhauer has their own web site and phone no. and are very easy to deal with over the phone for replacement items like sheaves. I found that Rig-rite has replacement sheaves for the Isomat boom, and they too have a web site. If you need more info regarding the web sites, phone nos, and/or equipment info, let me know.Ed