I started the summer after my sophomore year shadowing in a local outpatient clinic. I didn't do too many hours there and they were quite spread out through the summer, but it got my foot in the door and helped me start to explore other potential places around me I wanted to shadow at. I just emailed the owner of the clinic, since it was a private practice, and he said yes almost immediately and asked when I wanted to start.
I will say not every place is as easy-going is this. Two of my shadowing experiences let me come in to shadow as many hours as I wanted, no questions asked, but some had me fill out paperwork for background checks, immunizations, etc. but it all just depends on where you are shadowing (this was inpatient/sub-acute areas, which is why the screening was a bit more intensive).
My advice: It's great you're looking for advice now! I would definitely get a jump on it, it will really help relieve hour-related stress down the road when you are ready to apply to school so you aren't scrambling to find places to shadow at to get those last few hours and you can take your time finding places you really want to spend your time at and learn from. Most programs recommend or require 50-100 hours, so definitely try to exceed that. Try and find inpatient hours wherever possible, but they are hard to find if you don't already have a job within a hospital or rehabilitation hospital, as I know this was my struggle.
I hope this helps! Good luck!