I bought mine at a Architectural salvage company. I have also used the glass insulators from the old telephone poles, found them at antique stores.
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As mentioned above, any Farm & Fleet type store or an electrical supply house will have them. Some come with threaded screws in the base, some don't. The models with the threaded screws are called "cable hangers" or something similar. They typically come in two sizes with the larger ones working best for long runs. If you have a very short run or have relatively lightweight cables you can use the smaller ones.
One thing to keep in mind about these if you've got kids or cats / dogs that are around your system. They WILL trip over them, pull on them, yank on them, etc... unless they are WAY out of the traffic pattern. Just figured i'd save some of you folks the trouble of looking for these : ) Sean