http://www.stevespages.com/page7b.htmcovers damn near everything. The S&W 99 is specifically here: http://stevespages.com/pdf/s&w_99.pdf.
p.s. It's a good resource generally, but it doesn't seem to go into the detail you wanted. There's the set of field stripping instructions and then just the schematic diagram, with not much in between. Still, worth bookmarking, I think.