Find a local plastic supply store. The internet is a good start, but you should really just break out your good ole phone book and make a few calls.
What you are looking for is the following:
Cast Acrylic Rod (not extruded)
The size (in diameter) I have 3/8 in, 1/2 in, and 5/8 in
Realistically, you would only need something between 3/8 in and 1/2 in for support of any gunpla kit; so long as the length of support rod isn't excessively long. 1/2 in should be strong enough to support full resin/resin conversion kits.
Reason for "Cast" as opposed to extruded is the quality. You can drill and sand cast without the rod splitting under pressure or breaking apart.