I found these
Men's Women's Silicone Belt Rubber Plastic Buckle Plain Leather Style Adjustable | eBay
Plastic buckle, silicone belt. The buckle is the conventional dress belt sort of thing, I suspect it would pass for black metal at any distance beyond "intimate".
Or just the buckle
ABS PLASTIC BUCKLE Replacement Spare Silicone Belt Nickel Free Allergy No Metal | eBay
For the guys whose non ferrous belt buckles haven't been a problem at security, does that include the backscatter scanners? If I recall, even a plastic pen in my pocket was seen with those.