Trying to purchase the correct handlebar faceplate bolt in advance for Schwinn S2713TR My son has a Schwinn mountain bicycle and has lost one of the bolts to the faceplate that clamps the handlebar to the stem.

Now I dealing with this by phone, and I'd like to purchase a replacement on my way home.  But a trip home to unscrew the remaining bolt and then go to the hardware store to get a replacement can't really be done in one day before the store's closing.

Is there a standard part that typically used for this application that I can buy on the way home without having a sample to match?

Its a Schwinn Traverse 26 inch mountain bike model S2713TR with two sunken Allen head bolts holding the faceplate, one of which is missing
You really need to take the remaining bolt to the hardware store and get them to match something with the appropriate length and thread pitch. 

This isn't a standard length or size, but its likely M5 or M6 with length somewhere between 15-25 mm. You'd probably end up buying at least 6-8 packs of bolts to find the right one without measuring the existing one.