springs you want: http://intl-outdoor.com/gold-plated-beryllium-copper-spring-5-pcs-p-831.html
http://www.mtnelectronics.com/index.php?route=product/product&path=25_81&product_id=385
Been around long time, odd dimensions though. Richard notes they collapse somewhat, non-recoverable. Not much range.
Steel springs are coated with conductive metal like gold and silver, etc. How thick the coating, how thick the spring, all big factors.
These also tend to collapse: https://www.fasttech.com/products/1616/10001712/1347100-batterydriver-contact-support-spring-for-flashligh
djozz has other sources, did extensive testing: https://budgetlightforum.com/t/-/29825