Not that I'm aware of. Coxial cable by definition is circular. As the impedance is a ratio of inner conductor diameter and spacing to the outer conductor ( varying with the dielectric constant of the insulator ). Its physical size (appying ratios)can be reduced but losses will tend to increase and therefore, not useful for long runs.
Parallel flat feeders are available in 300 ohm, and I'm not aware of 75 ohm, it would be useless anyway, as no shielding.
So the short answer is nope.
Get some really good small diametre low loss cable from a specialist RF cable supplier, it will do the trick, but cost an arm and a leg.