The best online mattress that I've bought is the Casper Sleep Mattress that I bought on Amazon for around $1,000.00
It was well worth every penny I spent as it provides a comfortable sleeping environment and I get a good nights sleep on the Casper Sleep Mattress.
Some other good online mattresses that have good reviews are the Bear Mattress, Boll & Branch Mattress, Aviya Mattress, Amazon Basics memory foam mattress and last but not least the Allswell mattress.
Those all have great reviews for online mattresses.