I have had success ordering parts
for Isomat from Sparcraft in the USA, they are a possible source for you, but there may well be a more local source.
Splinting with 2X4s or 2X6s and lag bolts might tide you over until the replacement extrusion can be sourced.
Given your location, a search of riggers in the area (say Thailand
. . .) to check their boneyards may turn up an extrusion. Most riggers I have dealt with have a personal scrapyard of discarded bits lying out back of the shop.
Finding the precise technical description of the boom (look at Measurements and sections - Booms R.D.M. charleston-spar
) is a good first step. Don't forget to order the rivets at the same time, they can be hard to find in that size commercial
off the shelf.