If it's just the crank arm that is stripped, you might get away with just re-tapping the threads. This will only work if you still have enough material left to form new threads. You want either 1/2" x 20 or 9/16" x 20. Measure the overall outside diameter of the threads on your pedal to find out what size tap you need. You should be able to find one at a good local hardware store. If you're able to make this happen, it would be the most inexpensive solution by far. I'd recommend starting the tap from the opposite side of the crank from where you would normally insert the pedal. That way you have a better chance of picking up the good threads.
Otherwise, you'll need a new crankarm. If you can't find just one crankarm that's compatible with the bottom bracket you have, you'll probably end up having to buy a whole new bottom bracket and crankset. If it comes to it, post a pic of your crankset and maybe someone here can help you find what you need.
The first two sentences of your post were very hard to understand, by the way.
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In memory of Jim Price (aka. sydney) ...