You can try this place.
Also, REI sells stuff by the foot. I bought some 3/4 webbing from them, as well as buckles and ladderlocks.

Another source is old backpacks. If I have a pack or bag that I don't want to keep, I usually cut off all the hardware and maybe some strapping and keep those for the odd project later on. You can also try going to a thrift store and buying an old pack or something else that has the buckles you need.
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