Amazon offers Prime members the TROND G2 20-watt Dual Port USB Wall Charger for $10.99 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.10.) That's $9 off list and the lowest price we could find. It's compatible with all 5V-rated mobile devices.
Of note, you can buy it with the Trond Air 4,000mAh Ultra-Slim External Battery Pack for $21.98 via coupon code "KM7653MB". Prime members again bag free shipping. That's a total of $38 off list and another price low.