Amazon seems pretty reasonable just search 6-30 for straight blade or L6-30 for locking.
Try eBay as well. NOS (new old stock) often comes up. I bought all my receptacles and plugs there.
Keep in mind too that you can oversize the requirements. For example, my receptacles/plugs are rated to 600V instead of 300V as that's what was readily available (and cheaper) when I was looking to order. You can always go higher voltage or current than what you need.
Kal
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