IKEA plinth boards (aka kickboards) are packaged with the seal &
wordless instructions, so sorry can't help with the name ;-)

It might just be possible to obtain a seal ex IKEA. If, as most
likely, it isn't on the shelves then call in on the kitchen dept & ask
them if they can order one for you. On another kitchen item they were
able to explode the components of a package on screen & order an
individual item for me. You may need a sob spiel 'damaged while
fitting' etc, & you may need one of the local experts on their computer
system.

Otherwise look in yellow pages for kitchen component wholsesalers. A
few of the regular kitchen installers also have substantial parts
operation & supply other installers.

HTH