There was some open source MP's for SA
AlienX released some of his multiplayers' source like Online andreas and RPG Andreas ,but I can't find it anymore(all the links are offline)
Also,once I saw at IRC a link to GTA Rumble's source but now I can't find it and I don't know if it was real.
I think the SA-Party's developers released SA-P's source,but I'm not sure
I suggest you to contact the developers of these mods,maybe they give you the source