Embodiments of a pressurized gas supply are provided. In one embodiment, the pressurized gas supply includes a pressurized gas reservoir and a pressure reducer fluidly coupled to the pressurized gas reservoir. The pressure reducer is configured to reduce the pressure of gas received from the pressurized gas reservoir to a predetermined output pressure (PO). A gas mixture is held within the pressurized gas reservoir at a starting pressure (PS) and at a starting temperature (TS). The gas mixture includes: (i) a warming gas having a positive Joule-Thomson (JT) coefficient at TS and over the pressure range PS-PO, and (ii) a cooling gas having a negative JT coefficient at TS and over the pressure range PS-PO. The cooling of the cooling gas at least partially offsets the warming of the warming gas when the gas mixture is expelled by the isothermal gas supply to achieve a desired gas output temperature.