Do not even worry about that part. It's main function is to keep your pant leg from the chain. They do not attach well, and break off easily in rough riding. Kids break them off all the time.

Edit: It will also keep children from gashing themselves on the big ring.

It's possible that a bike shop would have one laying around they'd give you, or sell you for a nominal cost. Worth walking in and asking if you have a shop nearby.

I did some searching, and the best I can find at the moment are these from eBay, and shipping from China:

Bike Bicycle Chainring Sprockets Cranksets Guard Protector 48T 4BOLTS 120mm | eBay

Bike Chainring Sprockets Cranksets Guard Protector 42T 4BOLTS 105mm | eBay

And this one from Amazon:

http://www.amazon.com/Nexify-Univers...crankset+guard

Count the teeth in your main ring. The two eBay models cover 48 teeth and 42 teeth respectively.

Count the number of screws used. Four screws is common these days, but some older bikes require a five-screw pattern.

The Nexify guard from Amazon attaches to square-taper cranks. Your bike probably has those, but you may need to remove a plug to get at the correct threads. I favor the design of the two eBay models.

Don't spend a lot of money on this. Easier to just roll up your sock to keep your pant leg from the chain. There is no effect on the actual operation of the bike itself.