Here's some handy links and references.
You can find some information on getting your programming environment for the course up and running here.
Paperspace seems to have some free GPU compute but it is often in use. Their paid GPUs units are reasonably priced, a P4000 is about 51 cents an hour.
Google Colab has GPUs for free, but storage is not persistent and it can be a mild pain in the ass to set up with Julia. There's a guide on running Colab here.
Lambda has some very cheap compute. A reasonably powerful machine with a Quadro RTX 6000 is about 50 cents an hour (the cheapest they offer).
JuliaHub is a nice Julia option, but the GPU options are really expensive ($8 per hour!) because the GPUs are very good. I think if all you want to do is CPU-only compute, this is a pretty good option, but for learning it's probably too expensive.