Solid core power cords under 300$ - List of manufacturers ?

Virtual dynamics david 2.0 is a nice solid core power cord

If you want much of the benefits of solid core, but with flexibility?   Litz wire would give you that benefit.  Pangea combines solid core with litz.... both worlds in one cord.

Litz wire.
https://en.wikipedia.org/wiki/Litz_wire

Usually only wire under 18 gauge max diameter is solid.  Heavier that that in a solid wire is likely fixed location like hookup wire in a circuit or fixed AC wiring in your wall.  Many cable outfits call things solid copper core, meaning it is copper not an alloy, but not single strands of a heavy wire gauge. 

An insulated single stand of copper wire is considered a solid wire regardless of its’ wire gauge.

Example.

Insulated solid wire.
https://www.digikey.com/catalog/en/partgroup/solid-hook-up-wire/1915
.

Insulated stranded wire.
https://www.digikey.com/catalog/en/partgroup/ecowire-plus-series/1912

An insulated stranded wire/conductor is multi stands of bare single strands of wire grouped together under a common insulation covering.