Try Tempe Sales on Broadway. The only place I can think of off-hand that might have tan buckles. Other than that, your best bet is Supply Captain, as Ganef pointed out.
I can get them from my supplier, but there is a minimum requirement...like a couple thousand...and I just don't need that many. I get mine from Supply Captain or one of my gear vendors (as a favor, so don't ask who since they won't sell to you).