Does it need to be nylon? For different purposes, polyester or polypropylene work just as well.
Also, have you just been looking at online stores? Many in the bulk supply of textiles only list what online shoppers want, so you may have better luck emailing and asking for what you are specifically wanting, including an estimate of how thick the webbing should be. Thing is, you may need to buy a roll, not sure how many places still send samples, but its worth asking. If you are wanting gunbelt thickness, that might involve more than a roll, as it might not be part of normal stock, but worth asking directly about.