I have the queen version of this this mattress. Purchased from Amazon with Free shipping. Have had 8 months and am 100% happy with it. You can get the king for $310 or $500 depending on the thickness.
https://www.amazon.com/gp/product/B0134FY2O2/ref=oh_aui_search_detailpage?ie=UTF8&th=1
I have the queen version of this this mattress. Purchased from Amazon with free shipping. I have had it 12 months and am 100% happy with it. The 12 inch queen is $230.
https://www.amazon.com/gp/product/B0134FY2O2/ref=oh_aui_search_detailpage?ie=UTF8&th=1
Here is the exact mattress: https://www.amazon.com/gp/product/B0134FY2O2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
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I also recommend their bedframes if you don't already have them: https://www.amazon.com/gp/product/B075FFG9ZH/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1