You could go and work somewhere like a hospital, but you might find that (especially at 17), they don't let you do anything interesting because it's a real risk, and public/government run places don't like to take such risks. My friends who had placements in hospitals found themselves doing boring paperwork, and not really getting to do anything in labs, or even see inside operating theatres, or even get a feel for what their supervisors did.
On the other hand, those who had placements at small companies found that supervisors were willing to let them have a go at anything, although they were obviously under close supervision. I did my first placement in Year 10 at an engineering consultancy, and they had me specifying boilers for hospitals, and doing a structural survey on a collapsing railway bridge! My placement in Year 12 was even more involved, I did a complete structural and electrical survey on an old mansion in the Peak District. Obviously, all my work was thorougly checked, but I was actually doing the work! I got cracking references from both of those, and some really useful contacts too.
Anyway, back to the original question, where to try and get a placement!
Depending on your field of interest, you could do worse than look in the Yellow Pages for things like material testing labs, forensic labs, and engineering consultancies. These types of things might be more engineering-based than physics, but at your age (and I'm not trying to be condescending here), they might be right up your street, since you haven't really had an option to specialise in engineering yet. And they'd definitely not be your typical 'manufacturing' environment, which most people would agree are more engineering than physics. If you wanted heavily physics based things, have a look around to see if there are any local observatories around, things like that. Ideally, you'd be able to go and shadow a postgrad or postdoc at your local university physics department, but sadly few have the time for this kind of thing. Might be worth a few phone calls though!
I'll carry on thinking; it's a harder task than one might imagine!