There are so many different brands of filament and so many different places to get filament. So far we have bought all of ours from Amazon. Black Friday sales save you a couple of dollars as does buying in bulk. I have been getting the Elegoo brand as that's what Uncle Steven has recommended.
Now that you have your Bambu Lab P1S with AMS set up and ready to print, there are a few items you should start off with printing that will help make your printing experience better. Here are a few of the items we printed when we got our P1S.