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.
The Grad rocket is a Russian design and may be manufactured in Gaza or smuggled in from Russia. It is an updated version of the Katyusha used by Russian army fired them in massive barrages from truck mounted launch 122mm tubes during the closing days of the allied attack on Berlin. The launchers were nicknamed Stalin's pipe organ. It (Grad) is a free flight missile using only rear fins for guidance.
We used a similar system from landing craft during the "island hopping" in the Pacific. The saturation barrage was intended to clear mine fields and concertina wire.
Regards,
GtG