Forum OpenACS Q&A: Response to Continuing TCL Development / Beginning Java.
This is a difficult issue. The few hosts that do offer PG are somewhat costly. Furfly is, IIRC, still offering moderately-priced aolserver/postgresql/openacs hosting services.
I also suspect that if you find a host that supports apache/postgresql hosting, you will be paying a large enough chunk of change that you can sweet talk them into letting you install mod_aolserver, which will allow you to use OpenACS.