I found stainless T-nuts online at;
https://www.bolt.com.au/index.php?cPath ... _1644_1689
Cost for the 10-24 unc are AU$168.08 per 100.
I phoned these guys and they said they would supply smaller qtys (@ $1.69 ea) plus around $10 postage.
I use 10mm thick aluminium, (sourced from scrap), and cut to around 15 x 15mm sq.
Bore a suitable sized hole in the core then epoxy the blocks in place.
You can tap the threads before or after mounting into the core. ( I tap the threads after mounting as I can accurately locate the positions from the perspex template I use).
I source my taps & drills from ;
http://www.ejwinter.com.au/catalogue.htm
Look up under the "tooling" section.
Fossil