There are many sources of information about how to sit and what to sit on. A few internet searches will provide lots of guidance for effective posture and equipment or cushions to support good posture.
The following list of sources is offered as a place to start. We offer these without implying any product recommendation for items to purchase.
For some basic ideas about options for sitting, see:
There are many sources of cushions and sitting benches. If you cannot make your own, you can look at some vendors (in Canada):
Banyen Books and Sound: https://www.banyen.com
North Meditation: https://northmeditation.com
These are representative sources. Doing an internet search on ‘meditation cushions’ or ‘meditation benches’ will yield even more choices.
Remember, the most important quality of whatever you sit on is that it allows you to be COMFORTABLE as you sit.