I know a while back there was a thing with Israeli black market folk who were selling weapons, primarily small arms, to the Palestinians.
However, I don't think they were responsible for anything as large as a TOW and anyways, that'd be stretching the realm of possibility. The Saudis had TOWs at least as of Desert Storm, but the regime there hates the Shiites. Of course, this could be a case of who Saudi Arabia hates more...
Also, they could conceivably be weapons captured from our forces in Iraq (kinda like the M249 Zarqawi fired in his video for propaganda purposes) and smuggled through Syria (if there are only a handful of them). But I don't know if we'd have any reason to have TOW missiles in Iraq, and getting missiles AND launchers would be difficult.
Well, sorting through the wanderings of my mind, I think the Saudi route sounds most reasonable IMO.
Actually, over 4000 TOW missiles were fired in Iraq....
Interesting information here...
http://proceedings.ndia.org/5650/Cannon.pdf