I haven't seen the extension as a kit anywhere, you'd be lucky to find it.
This is a good example of how badly made regulations in the EU result in the strictest adoption any member makes. Bigger
tanks aren't prohibited by the TPD but some countries opted to limit tank size to 2ml, manufacturers simply make only one "TPD version" and it's the restricted tank size one, no matter if your country allows for bigger tanks or not.
Several member states allow for bigger tanks (eg Germany, France, Czech Rep.), you should try to order from there in the future (if shipping isn't too expensive).