I don't know anything about a zmax. But if it's a spring that's in the base cap. Could you not replace it with any similar spring.
It might not be the cheapest option, or the best. But would the spring from one of these fit. http://www.stealthvape.co.uk/electronic-cigarette-mod-parts/VAMO-SS-BATT-CAP
The spring does come out of the cap easily, as I've just tried mine.
If it is the hot spring for the battery, check the widest diameter and match for flashlight battery spring, also Ivapour-Elixir have hot springs here but I don't know if they would fit the Zmax