Here is just a sample of SOME of the US registered 727-100s
FedEx has 51 operating 727-0xxFs (various 020s and 100s)
Northern Air Cargo: 3
Ryan International: 5
UPS: 51 strong fleet of re-engined 727-0xx/100s with Rolls Royce Tays.
So, there are lots of -100s still out there, your best bet is to see them hauling cargo. I see them every time I go anywhere, you cant miss the DHL, FedEx and UPS haulers as well as the USPS planes which mostly belong to Emery I believe.