Scotweb do a plain one with the buckle type you are after:

http://www.scotweb.co.uk/products/gr...square-buckle/

Although there is nothing on their website I bought a sporran from http://www.leathersporrans.co.uk/ and they made me a belt (with a buckle as opposed to a plate) to go with it. I believe they make everything to order and by the look of their tooled sporrans (mine was plain) they certainly have the ability to make something to your specifications. It might be worth dropping them an email to see if they can help. I was extremely pleased with both my belt and sporran. A small bonus is that the price was very keen.