This video lists the cheapest Solar Panels in the world. The cheapest solar panel brands include Canadian Solar, Kyocera, SolarWorld, photon-Solar and Trina Solar of Australia. Prostar a Chinese brand is one of the cheapest in the market if bought in bulk.
The suppliers in Australia are Solar Juice.
For Asia its Alibaba
For Europe it is Bimble Solar
And for North America it is Alt E Store