Filtapac ?

Yes.  I use Filtapac all the time.  Very cost effective and highly recommended. As you say you need to purchase the replacement canister first then just the replacement filter inserts after that.

The filters inserts contain the charcoal in a mesh "sock" which is replaced by opening the base of the canister by twisting the bottom  by quarter turn, remove the old filter and discarding. Replace with a new one

Very simple and saves money.  Just remember to tell the dealer not to replace the filter at service time or they will discard yours.

Does anyone know if the "Filtapac " refills will fit into the Whale wf3000 canister.

The bottom of the wf3000 can be gently pried open to facilitate the removal of the old filter sock, thus enabling a new on to be fitted.