Radiators are usually a good grounding point and cold water pipes too, unless your house is quite modern and a lot of PVC plastic pipe has been used, in this case you will need to check if it is grounded.
Touching the case every few minutes is a good way to dissipate static.
Its more the movement of static electricity than its mere presence that causes issues, just connecting yourself to the case should be enough to equalize the static between you and the case and therefor no static will move afterward.
But if you want to be connected to earth.
- You really could just clip onto a plugged in PSU, there's no reason not to do this.
- Any hard fixtures like a tap or sink.
- Aforementioned radiators
- Any plugged in electronics, you could do it with a toaster.
- Burying the connector directly into the ground.