Plastic endcaps in WA?

I am looking for plastic endcaps for 1"x1" square tubing (.0625" wall if it matters). Just the simple, cheap, black plastic things that press into the
tube and cap it off. When I lived in AZ, these were easy to come by, probably because they are used on fences all the time. Up here in Seattle, I can't find them anywhere and nobody even seems to know what I'm talking about.
Any ideas? Even a website where I could order some would be great, but I can't find that either.
Thanks, Ian
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Wow. You guys rock. Thanks so much.
I was just in Home Depot over by the metal last weekend and totally missed them, but I wasn't looking. If they don't come through, McMaster-Carr will be perfect even if I don't need 100 of 'em.
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