I'm not sure where in the documentation they list available backends... But you can see it in the source here: https://github.com/langchain-ai/langchain/tree/master/libs/langchain/langchain/llms (for the Python version)
You could use something like a Huggingface endpoint, or OpenRouter if you want flexibility but something easy to use. There are services like RunPod or vast.ai which let you rent cloud GPUs by the minute and run whatever you like. These would be some of the popular services.
Edit: Here's the documentation page for the JS version: https://js.langchain.com/docs/integrations/llms/