Hello and welcome! As
@azure has suggested, it could be helpful to provide more information about you and your specific dietary aims, as this can help give more suitable answers. And just out of idle curiosity, what are you researching??
I use one of the original (more basic) Ketonix breathalysers, which I bought over 3 years ago. It's a bit of a hefty price now but it's a one off cost and should (hopefully!) last indefinitely. I used cheap and cheerful Ketostix initially but since these measure waste ketones being flushed out in your urine you may find (as I did) that once you're are in ketosis/keto adapted, they stop measuring as you're burning the ketones rather than excreting them.
I simply follow a very low carb higher fat diet (I started off by eating around 20-25g carbs a day to "kick start" then upped this slightly and gnerally keep to under 50g carbs now) and don't particularly use any specifically ketogenic recipes or supplements. There are plenty of suitable/adaptable recipes in our low carb area of the forum and a "What have you eaten today?" thread which can be useful for ideas.
Robbity