Many of the key journals are published by Taylor and Francis, and can be found by searching the publisher's database below:
You can use ProQuest One Literature to search by literary movement, i.e. postcolonialism.
Go to Advanced Search Select Primary Texts from the left side of the page.
Click on the to see the Scope notes about searching by literary movement.
Use the look up literary movement link to add postcolonialism to your search: