The main 4 monster books from KP are published under an OGL and are available on open5e , you don't get any lore, art, or advice on how to run the monsters without paying for the books, but you can see the statblocks.
The kobold press website also has a tool that can show you the environments a monster can typically be found in.
I would still recommend getting the books, especially if you want to use any of the higher CR monsters as the lore makes it a lot easier to understand the mindset the creators had when designing them.