I've found a way to buy cheap plinths from Bunnings... Well actually it's nothing new, just getting the news out in a way.
These little circles of joy are called "DWV PVC Cap Holman" from Bunnings. They cost at the moment $1.08 for the little 40mm discs, the 50mm cost $2.39 and the large 100mm cost $1.64. No idea why the larger one is cheaper. I've been told that these caps are used in your normal domestic home applications while another type, 'high pressure' caps are domed at the top.
I don't have pictures of the inside walls of the caps but they are about 1.5mm thick. Considering a lot of my minis are based on spray can lids with rather thin walls it's fantastic! Also if you are concerned about the mini being top heavy then plug the base with cheap air-drying clay or plaster of paris.
Last time I looked I mustn't have looked very hard, because I only saw the 50mm versions. Now I can't wait to use the 40mm versions.
The writing at the top will need to be covered or filed off. Easy task either way. Topless plinth could just be capped with a disc of thicker plastic card, or maybe used for water effects, depth and all that.
Also while I'm at it - other useful 'found' plinths:
Tuna Cans
Spray Can lids
Lids from large containers of drinking water
Toy Blocks (not one of my ideas)
Gardening Stakes (nice size, hardwood, just cut to height)
Wood Turning Blanks (square unfinished wood, some nice hardwoods)
Chair legs (another Bunnings thing)
Handles and taps
Ice block molds filled with hard plaster or resin (again not one of mine)
Replacement tool handles (cut to size, mostly thinking broom handles here)
Cladding simple wooden plinths with wood veneers (not my idea... I think)
Ditto with plastic cladding (measure sides, score card at side measurement, fold around)
If anyone has any other unusual sources of plinths please comment.