Where are the cash registers from the 1970s?

You may find some of them on eBay, though not as often as you used to. If you do find one, it's likely to need a lot of repairs, and will weigh so much that it's either "local pick-up only" or will cost over $100 to ship by freight.

Most of these 1950s-1970s electro-mechanical registers were junked, because they're not beautiful to look at (like the really old brass and nickel registers), and if you just put them into storage (as most shopkeepers did when they bought electronic registers), they'll seize up, and most of the technicians who used to maintain and repair them have long-since retired.

Mechanical cash registers are full of mechanical parts, which make them weigh over 100 lbs. each. And today's electronic machines cost far less than the mechanical ones did, and can do far more than the electro-mechanical machines ever could.