You can get 8/2 and 6/2 romex in solid core.@ mkgus OP
I'm pretty sure it is not manufactured with solid conductors.
Per the NEC as well as NEMA, single conductor insulated #8awg solid copper is the largest allowed building wire. Good luck finding any one that carries it. You may find some old #8 solid copper insulated TW or THW single conductor wire. I can tell you from experience it a bear to work with.
Be careful when buying 600V rated so called audio grade building wire. NEC says it shall be Listed by a recognized NRTL testing laboratory like UL to name one. Some AHJ (Authority Having Jurisdiction) specify UL only.
Jim
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