Can you still buy iPhone 6s???

No.

Introduced September 2015 (16, 64, 128 GB) September 2016 (32 GB)

Discontinued September 2016 (16, 64 GB) September 2018 (32, 128 GB)

The Apple Store currently sells:

• iPhone 8 / 8 Plus
• iPhone Xr
• iPhone 11
• iPhone 11 Pro / Pro Max

Amazon still sells them but they are either refurbs (“renewed”) or prepaid. Same with other retailers like Best Buy, Walmart, etc.

The only older model current available is the 8.