In reply to deadpoet:
It is possible to get the slider back onto those silly gliders, but it's incredibly awkward. Heat up the replacement in hot water first to soften it and try to force it on at the end of the rail - using something as a wedge to open up the glider helps.
Personally though I'd look to sew through the strap to create a non-adjustable replacement, they don't hold much weight so just hand sewing should be fine. Or a decent cobblers should be able to do it if you can source the parts.
Getting compatible squeeze buckles is (almost always) impossible, so most likely you will need to replace both sides - if you want to keep the rail I would advise cutting the webbing off the glider and stitching new on with the glider still on the bag (as above, fitting the glider back on the rail is not easy).