Try Old Glory, they have (or had) some suitable guns. (as for crew they hace a bag with naval artillery crew, various headgear, so suitable for german, french, austrohungarian, belgian etc)
Also Eureka has a nice colonial gun and crew. Since they sell it separately you can buy as many crews and guns you like. And you are in Australia so, you can obtain them directly.
The other option you have is to try those model boats fittings. Some have suitable weaponry (Example: Sirmar models, Cornwall model boats and many more in UK
Sorry, I do not know in Australia but I bet there would many model shops with boat fittings. Anyhow there are lots of online suppliers.)