How to make a Cheap HDR Light Probe for Compositing
Wal-Mart has 8 inch Silver Christmas balls for sale right now for about $2.50
Find one with a smooth paint job and relatively few ripples on at least one side.
Using masking tape and plastic to mask off half of the ornament, spray paint the other half with auto primer.
Mount on a dowel.
This is as cheap as it's going to get, so I'd hop on it if I were you!