Setting up your own is not that hard and if you bought some compute before the Altman squeeze, very cheap.
+1 for microsandbox. I've been using their golang SDK (https://docs.microsandbox.dev/sdk/go/sandbox) @v0.5.10 to create sandboxes, attach them to agent sessions to execute, and then throw away, all in a raspberry pi 5 k3s cluster (as they have ARM support, if you're into that sort of thing). The microsandbox code is still a bit in flux (since it hasn't reached v1.0 API stability yet), but it's definitely worth checking out as it looks to have a solid foundation.
(edit: ahh sorry, meant to post this to above comment)
Yep I've got one I built and it's absolutely fine for my use cases has a web interface/API custom kernels and rootfs, even the facility to set-up custom Kubernetes clusters. It's been really useful for other work like testing out vulnerabilities or security features in isolated envs.
Def!
My personal belief is that the future of an "app" is a combo:
So, it should be stupid simple to run these local sandboxed apps/agents. Right now, not too hard for technical users (esp. with things like https://smolmachines.com/ and https://microsandbox.dev/), but not as easy as clicking an app icon or typing `/path/to/binary` in the CLI