I'm not familiar with the RSF3. I think that's their new helmet. In any case, their most popular models, the RSX and RSR2, shared visors but only between those two models. The RSF3 being one of their newer lids (I think it replaces the RSX) uses a completely different latching mechanism....and also uses a thinner visor.
Source:
http://www.webbikeworld.com/r2/motor...t/shark-rsf-3/
Anyway, as previously mentioned, your best bet is sportbiketrackgear.com. Here's a link:
http://stores.sportbiketrackgear.com/Detail.bok?no=4540
Edit: yup...it uses a 2.2mm visor vs the RSR2/RSX 3mm visor. Damn shame really....