I bought a 40 dollar chef knife on Amazon the same time I bought the stone. Turns out it's a very good knife. When I was learning, I also sharpened every shitty knife I owned and they were like brand new.

Just practice on whatever you have. I never ruined anything with the whetstone, only with the cheaper sharpening tools.

You're not going to be removing a ton of material at once with a stone, so personally I don't think it's a high risk thing to attempt on your own knives. (That said, YMMV and please don't come after me with said knives kthx.) Just don't do anything crazy, like running the knife 90 degrees to or along the corner of the stone, and you'll be very unlikely to ruin anything.

You could also see if any thrift stores or buy nothing groups have a knife, then you can practice on it.