A chemical weapon can be in any form. Naturally, it's handier if it is packaged for end user; however a simple gas cylinder with a grenade around the valve will kill just as well.
Additionally, if the gas is distributed in unmarked, generic cylinders then it's harder to associate it with any specific source - like Saudi Arabia, for example.
I tend to think If insurgents were being supplied with this stuff, it would be in the form of artillery shells, mortar rounds, or perhaps rockets.
Rebels, as I understand, don't have much of artillery or missiles. What they have may use a wrong caliber. Saudis are armed with US weapons; Syria is armed with USSR's weapons. The rebels' sort of delivery technology would be a junky Toyota pickup, loaded with a few of those gas cylinders and a small explosive charge, driven by a suicide bomber. Another option is to leave the chemical weapon behind during a retreat and activate it remotely, once the enemy forces are in range.