That is really hard to answer. Wikipedia says "Over 80" Cray-1s were sold, so they weren't all that common, despite being the iconic representation of a supercomputer.

The answer is different for each machine. There are emulators for some of the older machines. See http://simh.trailing-edge.com/ and http://simh.trailing-edge.com/links.html for some.

Here's an HP-2000 emulator:

1. Telnet to mickey.publicvm.com (yes, telnet)

2. Slowly alternate CTL-J and CTL-M until you see a “PLEASE LOG IN” message.

3. Enter “HEL-T001,HP2000,1″ . The resulting output includes:

    Try exe-castle.b300, exe-kingdm.g100, exe-spawar.r500
    To play the 1975 version of Oregon Trail type exe-oregon
    To play the 1978 version of Oregon Trail type exe-ore2
    To hunt the Wumpus, type exe-wumpus

Then "exe-oregon"

  DO YOU NEED INSTRUCTIONS  (YES/NO)?YES

  THIS PROGRAM SIMULATES A TRIP OVER THE OREGON TRAIL FROM
  INDEPENDENCE, MISSOURI TO OREGON CITY, OREGON IN 1847.
  YOUR FAMILY OF FIVE WILL COVER THE 2000 MILE OREGON TRAIL
  IN 5-6 MONTHS --- IF YOU MAKE IT ALIVE.

  YOU HAD SAVED $900 TO SPEND FOR THE TRIP, AND YOU'VE JUST
     PAID $200 FOR A WAGON.