If it is a bicycle, shouldn't be to hard to find a replacement. Most bicycle chain is 1/2" cc of the pins. You can use your caliper to measure it. Or more accurately measure 6 double links while stretched. Worn out chain is too long when stretched.
Then check the width. "5 speed" or "bicycle chain" is 1/4" wide. 6 7 8 speed chain is 3/32" wide. 9 10 11 speed chain is thinner yet; I don't have one so I don't know how much. Again use your caliper to measure outside width. Count the links; mountain bikes tend to have more links than a standard length chain. For my stretch cargo bike I had to buy two chains and an extra master link to splice them.
I buy KMC chain which lasted me 5000 miles but there are other brands. Modernbike.com and universalcycles.com are big internet houses that actually stock items.
No caliper? Starrett makes the best but fowler is okay. Mitutoyo is in between but electric, which causes battery damage in low home use. mscdirect.com will sell them. I got a decent fowler off e-bay last year after my previous one was stolen.