Summary: Replacing a buckle is trickier than you think. They all look alike, but there are subtle differences that affect performance. Only the manufacturer can be certain you are getting an exact replacement matched to your strap material. We have a page on how to thread a buckle on.
The best replacement is from only one source: the manufacturer of your helmet. There are reasons for this:
So we recommend as a first step that you contact the manufacturer. They did, after all, sell you this helmet with a cheap plastic buckle. In our opinion they have an obligation to make sure you can find a new buckle, even if the warranty has expired and they charge you for it. Without their customer support your helmet is essentially useless, as they and you well know.
If you can't find the manufacturer, or they don't have a replacement buckle, what to do? You can look for an identical helmet with an Ebay search or perhaps on your local Craig's List or even at a yard sale, and scavenge the buckle. There are lots of used helmets around, but for the reasons explained above you need an identical helmet to be sure the buckle is the right one. That's a tall order. And there may have been "running changes" in the materials in your helmet model over time, so you are still not sure you have an exact replacement.
Taking buckles from other brands or models of helmets is probably even more dicey, since the buckle must be closely matched to the strap material. Wal-Mart and Target have helmets for $10 each, but again the buckle on one of those may or may not be right for your old helmet. Sewing a new buckle on requires just the right thread and stitching, or it will deteriorate with age and may not hold when you need it to.
This Google search for Fastex buckles may turn something up. But what we found was a supplier who said the buckles were not for critical uses like helmets. We did find one Echo brand replacement buckle on the web, but it is designed for motorcycle helmets. It will not work for your bike helmet, since it is designed to replace the D-rings on a motorcycle helmet and requires a pre-sewn D-ring loop at the end of the strap. It is also designed for a much wider motorcycle helmet strap, and would skew sideways under tension with a narrower bike helmet strap.
The last thing you want to hear from us is that you may have to replace your whole helmet because the buckle is
broken. But you can't continue to ride with a broken buckle! If one prong is broken off, or any piece of it is broken
off, that buckle is an illusion, and the helmet can fly off your head at any first jerk or impact, leaving you bareheaded
when your head cracks on the pavement. You can keep the old one on a shelf, labeled BUCKLE BROKEN DO NOT USE in hopes of
finding a buckle somewhere eventually, but go ahead and buy a new one before your next ride.