If it was this weekend you would have no problem finding a boom there.....
www.merimbulasailboardclub.com.au/Maybe your best bet is to contact the Merimbula sailboard,club

Try Facebook page, they may have a spare in the clubhouse or steer you in the right direction. Dont forget to show your appreciation if they help you out, they are probably resting from the last Merimbula Classic ......and planning the next.