You can find keras2ncnn tools here

https://github.com/MarsTechHAN/keras2ncnn
https://github.com/azeme1/keras2ncnn

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From tensorflow 2.x, you can also export mlir and use mlir2ncnn which is maintained by the official.

The source code is located here: https://github.com/Tencent/ncnn/tree/master/tools/mlir
For Chinese, you can refer the guide here https://zhuanlan.zhihu.com/p/152535430