Just looked at them, they appear to be generic brand....
REI or any similar outdoor store has really good selections of nylon webbing in the outdoor rock climbing section, and usually has a good selection of buckles available for repairing outdoor gear. They're often found nearby, or with the backpacks... Take a weight with you if you want to assess for fit, or just consider cutting off the clip that's there and replacing it with a bit of strap and a new set of fastex buckles. They're about $1.50 or $2 a pair in the size you would need with ankle weights.
You can pretty easily sew two layers of nylon webbing on any decent (Singer, Bernina, Viking, Husqvarna, etc -- just not your $99 Wal-Mart special) brand of home sewing machine, usually three layers on superior grade ones, and anything thicker more or less requires an industrial machine or use of a hand awl. Use thick upholstery thread - not cotton, but nylon - ask the saleslady for upholstery thread from Gutterman or Coats & Clark - and the appropriate needle, which is usually a denim needle in a 100 or 110 size for that thread.
I tend to sew a straight stitch "X in a box" pattern on the end of the webbing, then bar-tack over the ends of the stitching (use a buttonhole stitch) to lock the pieces together... I think the anal-rentitiveness in securing the webbing comes from dealing with my parachute/skydiving gear...or the rebreather counterlung tie-downs, which I made in the same way. Either one failing is a serious problem, unlike losing an ankle weight...
What you want to do here should take you no more than about 15 minutes when you're gathered the supplies.
That all said - if you're using ankle weights -- wouldn't it be easier to figure out why your trim is so off that you need them in the first place? Correct trim in your gear should relegate the ankle weights to permanent status in the "swamp bucket" in the corner of the garage...
