I would recommend trying to shift the tranny by hand on it with the cable disconnected. Next, try moving the cable on its own with nothing connected. Then try shifting it with the cable connected to the tranny but disconnected from your shifter. Then hook up the shifter and see how it goes. The force is not going to be the same for each of them since there is mechanical advantage but it will give you an idea of how easily the cable moves.
I have seen several things cause this problem. If the rigid ends of the cable get bent at all, it will be hard to shift. If the cable itself is corroded, it will be hard. Another common thing that I have seen is a worn out shifter (I have not had good luck with the morse units). There are a lot of things that can cause the problem so I would suggest troubleshooting first. Cables are cheap and not a big deal to replace but while you are at it, you might as well fix the problem.
If changing cables, you might consider something other than the Teleflex/Morse units. I have heard really good reports with the cables from Kobelt. I have no personal experience with their cables but have had very good experiences with their shifter and I would expect that there cables are well built as well.
To answer your original question, just pull the old cable and take it into the store with you. If it has a hard to reach run, you might want to pull a line through when you remove the old one to allow you to pull the new cable through.