Wargames Factory are now selling their plastic bases on their online store.
From their website:
We’ve added a new section to our web store; “Individual sprues and Bases“.
Initially available in this section are 3 new plastic bases to improve the look and stability of your figures. We’ll be steadily adding sprues from all of our various kits to this section as well. So if you are needing spare parts or looking to mix sets and kit bash keep your eyes on our new store section.