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Amazon [amazon.com] has Klein Tools GFCI Outlet Tester for $10.98.
Shipping is free with Prime or on $35+ orders.

Lowe’s [lowes.com] also has Klein Tools GFCI Outlet Tester (RT210) for $10.98. Shipping is free for MyLowe’s Rewards Members (free to join [lowes.com]) or select free store pickup where stock permits.