Given your Java inquiry was 1/2400, I'll skew in that direction.
Your first stop should be Viking Forge. They've got an extensive line of 2400 scale ships, including a WWI Generic category with gunboats and other small craft.
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Viking Forge also has a Spanish American War and Russo-Japan War section you should peruse.
Your second stop should be Panzerschiffes. They make rather primitive resin cast models, but at the gunboat size level they work fine.
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You'll want to look thru pretty much their whole inventory as there will be WWII small craft that could well pass for earlier gunboats. Check out D095 River Class UK. Also older types. Span-Am, Russo-Jpn, Sino-Jpn.
Your third stop should be Wartimes Journal. They are a 3d print company producing high quality pre dread models.
wtj.com/store
wtj.com/product6064.html
They can print at several scales. 1/2400 is almost always an option.
From the UK consider Tumbling Dice 1/2400.
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reveals a long-lost Martian tripod.
