If your class is one of the dual wielders, i.e. Sentinel/Marauder/Gunslinger/Mercenary, you can do HM Battle of Ilum to get the token for columi grade off-hand weapon and pull out the barrel/hilt to insert into your mainhand weapon.
If not, the only flashpoint where you can obtain a columi mainhand token is HM Lost Island.
Also works great for the single weapon alternative advanced class - my scoundrel picked up an offhand token in Battle of Ilum (Hard) and turned it in for a gunslinger offhand weapon and then stripped the barrel.
I also got a columi weapon for my vanguard from Lost Island (Hard) last boss and for my shadow from Karraga's Palace (Story) last boss.