Anyone can replace a broken car window crank. If you find that thing that you grab when you roll down your car window to be broken, don't race off and waste your money at an auto repair shop. The local car mechanic is not the answer here.
Many car window cranks can be found at auto repair stores. There is typically a section of the store dedicated just for these handles or cranks. The come in a variety of colors or sizes. If you find that the store does not have the 'roll up' handle you need, they can typically order one for you.
You might be able to save some money by going to a junk yard and finding a new or definitely used handle. For an item this cheap though, you might wind up paying as much at the junk yard as you would just ordering it new yourself. That said, some junk yards are as sophisticated as many auto parts stores with computer indexed inventory, new parts, good service, etc.
Another option, and I have replaced two window crank handles using this method, is buying them on line. I bought both of mine on eBay but there are other sites out there as well that can provide you with a door handle crank to fit your exact needs.
When buying a window crank, you'll need to remember a few things. I'll list them below. If need be, take some notes first before going to the store or checking online. Here they are:
1. Know the make of your car
2. Know the model of your car
3. Know the year of your car
4. Check out the color of your old handle. Actually... this is important. I swore I had a gray window crank only to discover after I bought it that I really needed a black crank. I put it on anyway, my car is really old.
If you want to learn how to install the window crank, I did an article on this very topic and linked it here as well as below. Check it out!
Repair and Replace Window Crank Handle on Car Door