If your in need of a real leather buckle try ATbutton.com
Trench coat belt buckle replacement? - Page 2
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Got a vintage Burberry from E-bay recently, and it looked great but for the condition of the buckles. I e-mailed Burberry through their main site, and while they did ask when and where I got the coat (Claimed ignorance!). The agreed to sell me replacements.
But unfortunately they weren't cheap.
A well-sorted haberdashers (tailoring supply) should stock them.
Replacing a buckle is an easy job (can be done by hand).