Prices on the Galaxy S9 are starting to come down, even if you're going through carriers. However, the carriers won't sell you an unlocked dual-SIM version of the phone. On eBay, you can pick up the dual-SIM GS9 with 128GB of storage for just $639.99. You can also get it in any color you want, as long as that color happens to be black.
The SM-G960F/DS is similar to the Galaxy S9 you can get at retail in the US. There's a 5.8-inch OLED panel, 4GB of RAM, and 128GB of storage. However, instead of a Snapdragon 845, this phone has an Exynos 9810 SoC. You can drop two SIM cards in this device, but there's no CDMA support. You'll get a plethora of LTE bands that should cover all major GSM/LTE networks, though.
We've seen this version of the phone pop up for sale several times in the past, but this is the cheapest it's ever been (especially with 128GB of storage). You only have to pay tax on the purchase in Texas, and shipping is free in the US. The seller will ship to various other countries if you're willing to pay for it.