Champions Choice does sell the carbide in plastic bottles. Caving supply catalogs are also a good source.
If you have a welding supplies store near you, ask them if they sell it. Most don't unfortunately, but if they don't, ask them if there is a bottle plant around that fills acetylene cylinders. These places use the carbide by the barrel. I got a huge bag about half the size of a coffee can from a bottle plant. I asked the guy how much and he said "don't worry about it. I'll charge you double next time".
I have also used the spray on type and don't care for it. It is not as black.