I bought it here:
https://www.gereedschapcentrum.nl/bostik-superfix-009-lijmkit-grijs-patroon-290ml-30606093.html?gclid=EAIaIQobChMIzMPFiu6O5QIVRYXVCh39wQpkEAQYBSABEgJylPD_BwE
This is a dutch website, but I think they also ship to Belgium. Alternatively you can check bol.com. also a dutch site but shipping to Belgium is very often possible.
THis works great. I also use it to seal the 3 phase wires going through the AFT mast mount. Super water tight and remains quite flexible
!