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Are you talking about the little slotted tool that unlocks the front sight blade, or the U shaped tool that drives the sight blade left and right? If the slotted tool, you can make one out of a screwdriver with a dremel, or you can buy them premade at NAPA for tire valves.
If the U shaped one, there are replicas on ebay for around $50. I had a bunch of originals, but they sold quickly at gun shows for $85 each.