where can I find high gauge copper wires for an homemade induction heater?
Tubes (pipes) are used for these things.
First, you run the thing at frequencies where the current flows in a thin layer and doesn't care if the conductor is hollow (and it may as well run on the inside, halving the resistance).
Second, it may be practical (or outright required) to run coolant (usually water) inside.
The places where you can get these tubes for cheap:

Air conditioning suppliers and workshops. You may even get shorter pieces (1m or less) at scrap prices or for free. 6mm, 8mm, 10mm, 12mm and their corresponding imperial sizes widely used.

LPG car conversion suppliers / workshops. LPG conversion is popular here, may or may not be popular where you live. Copper tubes have fallen somewhat out of use, but are still used. Tubes come in PVC insulation (just like a cable) that may or may not be useful in your case. 6mm and 8mm outside diameters are common, scrap pieces common as well.

Hydraulics and pneumatics suppliers / workshops (including, but not limited to, car and truck brakes).


Some of these places distinguish &quot;soft&quot; and &quot;hard&quot; pipes. You need the &quot;soft&quot; ones, &quot;hard&quot; are way harder to safely bend.
You can as well grab tools for aesthetically bending these tubes from the same places.
p.s. plumbers use 12mm and larger, but it is the &quot;hard&quot; variety (and you will rather not make an installation that big).