I used the standard Scottish thistle pattern buckles. I got them off Ebay.

About the sort of shoes which work for making up into "buckle loafers" I like starting with tassel loafers with wingtip style decoration. I've found these, very cheap, on Ebay:

Allen Edmonds "Chambrey"
I think it's ideal for affixing buckles to. Nice wingtip decoration, and a strap across the top for the buckle to be placed upon. All that has to be done is remove the tassels.

Allen Edmonds "Hardwick"
This shoe is very similar to the Chambrey but lacks the strap. So once you remove the tassels there's a large plain area where the buckle will go.

Nunn Bush
I don't know the name of this style but it's a wingtip tassel loafer with decorative strap over the top and fancy serrated flap. I removed the tassels and cut off the flap before I affixed the buckles.