There are tunnels all over the place. I hear some are from Iran. Some are sent by ships.
Iran sent some of them. The “sewer pipe” that was openly delivered was turned into rockets so all of the above I guess.
Here’s the answer if you want to subscribe.
http://www.nbcnews.com/news/world/hamas-firing-china-designed-syria-made-m-302-rockets-israel-n152461
In essence there is always somebody willing to make a buck. Nicolas Cage played an arms dealer in (I think) Lord of War. Essentially, that explains how it works.
Many times other countries are playing at proxy war. Then there are countries like Russia that have zero control over what gets exported so long as the right officials are bribed. Most likely the rockets are from Russia, China, North Korea and/or Iran. All are lawless wild-west zones where a buck will get you anything you want.