Any local bike shop should be able to make spokes for you.
If there aren't any, then there are websites like Grin Tech http://ebikes.ca (and others around the world) that can custom cut spokes for you.
To find out the length you need, you can measure one of the unbroken spokes on the same side of the wheel (if it's dished, it's important to be the same side), or you can measure the two parts of the broken one. You'll also need to know what gauge that spoke is, whcih the place you order from can show you how to do, or the place you take it in to if local can measure that.
FWIW, if spokes are breaking, it's usually because the spokes are not tensioned sufficiently, and flex at the wrong points. The fix for that is to retension the spokes to whatever is correct for that gauge spoke, when retruing the wheel after replacing the broken one. They may require retensioning again after riding it a short while. If they continue to need retensioning, then see below.
Or they *were* tensioned correctly, but the tension was so high that it cracked the rim around the nipple hole. The fix for that is either replace the spokes with ones thin enough for the replacement rim (if identical) that's required to fix the cracked hole, which will now tension correctly without damaging the rim, and make a stronger wheel, or to replace the rim with one capable of handling the tension required for the thicker spokes.
Sometimes it's because there is too much of an angle at the nipple entrance, and the spokes are bent right at the thread start, which then fractures along the thread line from stress. The fix for that is to angle the nipples with cupped washers inside the rim (and potentially drilling out the nipple holes in the rim a bit) so the nipples are in a straight line with the spokes.
