I'm needing to get about 20 13 round USP 9c / P2000 9mm 13 round mags.  I'm not up to speed on where to get H&K stuff.

A lot of good stuff on here, and for those mage they are cheaper than Midway



HK Parts

Its been a while, but CDNN used to be priced ok.

https://www.cdnnsports.com/magazines.html#manufacturer=HECKLER%20%26%20KOCH&caliber=9MM

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CDNN has the best pricing based on my recent searches.  They had the 13 round variants for $24.95 a few months ago but raised the price to $29.99.    I got some of the 10 rounds versions of that particular mag as my beat em up range mags simply because at the price of $19.99 at CDNN that is an extreme bargain for an HK factory mag and I feel better about beating them up in practice use.