Do you mean libraries for backend languages, like Go
and Java, or servers, like Nginx and Caddy? Because for Go
there is the quic-go module[1], and since Caddy is written
in Go, it already has (experimental) support for HTTP/3[2].
Envoy proxy has initial support using Google's Quiche. Docs and example configs are being worked on. I wrote up some h3 client and Envoy proxy testing this week [0].
Depending on what you want to accomplish, an option is running your services in a cloud that supports terminating QUIC on their front-end load balancers. Google Cloud does, for example.