Best place to find big rocks

I know it sounds specific and dumb but go with me on it please.

I’m looking for some medium size (like potato size) rocks for a project. Ideally one I can drill into without it falling apart? The more unique the better.

I really don’t want to go to some plant shop and be charged $20 a rock for something I’m sure there’s a park within 2 hours and an enjoyable hike that would have it.

Any suggestions?