Amazon looks like your best bet:

https://www.amazon.com/Vegan-Collection-Garrison-Black-Small/dp/B00O3GVOLG/ref=sr_1_1?s=apparel&ie=UTF8&qid=1505107301&sr=1-1&nodeID=7147441011&psd=1&keywords=The+Vegan+Collection+Garrison+Black+Men%27s+Belt

It's a brass buckle. I've suspected that I've developed that same allergy to nickel in the last year or so and am amazed at how often jean buttons and belts buckles are made from it. (Not to mention how often you can't find the info saying what the metal is on the clothes.)