I'm assuming that you're currently running a standard 175mm crank arm length. I don't know what the guys at your LBS told you, but worst case scenario you have to install a new bottom bracket, and that may be included with the crankset you decide to purchase. Good luck finding anything shorter that isn't a single-ring setup.

I once ran a 170mm double on a 7" AM/DH bike. Not for knee issues (which I have), but because I was experiencing excessive pedal strike. I honestly couldn't tell the difference between them and the 175mm cranks on my other trail bike.

This is an interesting article on crank length.
http://powercranks.com/cld.html
The crank length-seat height determines the knee and hip angles when the cranks are at the bottom and top of the stroke. The crank length-handlebar distance determines how close the knee comes to the chest, especially in the aero position.

This is a good discussion from some who are experiencing your knee problem. Read through and hopefully it will help. I agree that you need to be sure your saddle height is correct and then try the shorter arms. Your local bike shop guys are probably young and not knowledgeable how to help the older cyclist. Reading these two pieces has given me cause to rethink crank length. I have not had knee surgery but have issues especially in my left knee. Since I started cycling again 40 - 50 days ago I am experiencing little or no pain. We will see what November temps will have to say about that.
http://www.bikeforums.net/bicycle-mechanics/1008740-shorter-crank-arms.html

Here's another forum.
http://forums.roadbikereview.com/general-cycling-discussion/hip-flexor-pain-crank-arm-length-318811.html

Here's a 165mm crank.
http://www.amazon.com/Redline-Microline-Crank-Arm-Set/dp/B0039VB3G2/ref=sr_1_8?s=sporting-goods&ie=UTF8&qid=1436632937&sr=1-8&keywords=crank+arm+165mm

And a 160mm
http://www.amazon.com/Redline-Microline-Crank-160mm-Black/dp/B0030KXESC/ref=sr_1_2?s=sporting-goods&ie=UTF8&qid=1436633034&sr=1-2&keywords=crank+arm+160mm

Crank arm shorteners that you mount to your existing arms. Looks like some folks with knee issues are using them successfully. Someone even mentioned using them with a Trek hybrid like mine. Need to read up a little further...
http://www.amazon.com/Ride2-Crank-Arm-Shorteners-43mm/dp/B002N2KJ7W

This way you could start at the shortest length and see how your knees feel and work up to get more torque. Looking at them they might kick the arms out a bit wider which would be a plus for me. I think that I am touching the left chain stay with my heel at times (I have to be slightly toe out to prevent knee pain). I worry that I will do damage to it and the shoe. My old left shoe was pretty scuffed up.

One plus I can see to shorter cranks is that you will be encouraged to spin at a higher cadence. Personally I find spinning 70 to 85 RPM really helps my knees.