Go to Home Depot or Menards and look at the dryer cords. Make sure the 3 prong on the dryer cord is the SAME as what you have in your garage, now. There are a few different 3 prong styles. If they DON'T match, I would just do what Rick is saying and just change the end.
These guys walked me through what I did. I like to mess around with this stuff and learn about it. If you are not real handy, I would do what Rick is saying. Also available at Home Depot or Menards.
If it matches. You can buy that one and remove your cover on the heater which is held on by 4 little screws. You will see the wiring it hooked up to 3 places. You will have to cut a few plastic wiring straps that hold all the wires together. Super easy.
You'll see a green ground. a white and a black wire (Your 2 hot wires.) Replace those wire ends (they are "slip ons" that your heater needs) becuase the dryer cord comes with 'round" ends. Snip them off and replace with slip on ends/tips.
The heater comes with 2 hot wires (white and black) and a ground (Green). Your new dryer cord will have the same.
Here are pics of mine before and after.
BEFORE- You see the white and black wires. They are your HOT wires. The ground is not shown as it is just screwed onto the cabinet for grounding BEFORE the picture starts on the left hand side.
AFTER- Black to the old black, but red to where the original white one was. I think on a 3 prong, your wires will be black and red or black and white. Either way they both are HOT wires. Just put one or the other on each of the areas. Green to original ground spot.