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I found the Nerf Rival 18-Round Refill Pack and 12-Round Magazine on sale for $ 6.96 on Walmart online. Linky [walmart.com]

This is cheaper than:
Target $ 14.99 Linky [target.com]
Amazon $ 11.06 Linky [amazon.com]

I also found a Nerf Rival 18-Round Refill Pack and 12-Round Magazine locally for $ 1.00 so it can be found cheaper in store than online, but that is YMMV.