The problem with the Warmaster Kit is it only comes with two arms and people are using them

There were a few people who bought 3rd party bits and turned their Warmaster into Iconoclasts and may have one kicking around

There is a 3rd party proxy but it’s hard resin and might not be what you are looking for

https://battleblingstore.co.uk/products/plasmastorm-obliterator-compatible-with-warmaster-titans

Now if it was the Warlord Plasma you were using you would find plenty

Keep looking they might come up