It sounds like you're looking for a leather belt strap with press studs for a pin buckle, specifically in a width of 38mm and in either black or brown color.
Craft and Leather Supply Stores: If you prefer a DIY approach, you can purchase leather belt blanks and press studs from craft stores or leather supply shops. This way, you can make a custom belt strap to your exact specifications.
Custom Leather Crafters: If you want something truly unique or have specific requirements, consider reaching out to custom leather crafters. They can create a belt strap tailored to your preferences.
Remember to measure your buckle's attachment points to ensure that the press studs on the belt strap match the buckle correctly. Additionally, double-check the width (38mm) to make sure it fits your belt loops and buckle.