Good grief, that bike shop should be nuked for giving you 175 mm cranks. Here I am wondering if 175's might be a bit on the long side for me (at 5'7" and 32-ish inseam) - they were certainly too long for you. Those long crank arms could not have helped your knees one bit !

As for finding "hard-to-find" size cranks - many cranks do come with different crank arm lengths, but since it's clear some bike shop nits (er, employees ) are too darn lazy to waddle over to the shelf and open up the catalog, or to find the manufacturer's information on the web, sad but true it's up to you as the Informed Consumer to find out that information for yourself and shove it under the nose of (uh, nicely present it to) said lazy nit (um, helpful bike shop employee).

That said , often it's the higher-priced cranksets that come available with the variety of crank arm lengths.