When you mention 3/8" or 1/4", are you measuring across the "face" of the material, or you may call it the long side of the triangle? I got some leather fillets from Bill Sawyers widow a few years ago, but can't find her contact info. Maybe a search here or on SSW will turn it up. I believe it is still available for the original use it was intended for in the tool/die/casting industry.
Type at you later,
Dan McEntee