Where does everyone look for cool sounding props? If you do, do you have places you check on a regular basis?
For me its mostly flea markets, friends storage places ("Do you need this?") and garbage bins.
Your house! Kitchens and garages are particularly useful. Need a metal slider on a prison door opening? Try sliding your oven racks in and out. All of the best arm whooshes you'll need are just serving spoons and box graters! Don't discount what you have lying around. Walk around your kitchen with your headphones on and your handheld recorder and see what types of sounds you hear that you don't hear with your normal ears. We're way too close to hearing those sorts of things, the headphones and mics really put a different spin.
Also, in Ric Viers SFX Bible (you should buy this immediately), he talks about dollar stores and thrift stores as being one of his favorite resources. Especially when you're talking about things like glass debris, no one wants to spend a ton of money on glass to break, so he actually asks them if they get any broken glasses to save and he buys them at a discount (i'm paraphrasing that whole part, but you should read it).
Outdoors and public places in general, flea markets, acquaintances with jobs in interesting places (and thus have access to them) and roles or that have "stuff"/gear lying around, landfills.
For a recent radio drama, I had great luck at my local creative reuse center. I found it through my local craigslist crafters forum. Here's a list of reuse centers. You might want to see if there's a local maker/crafter hobby group, and ask them about resources.
Goodwill, Salvation Army, Dollar stores were good resources. Also keep an eye out on craigslist and freecycle. I also searched yelp for home decor stores. Most of them were out of my price range, but a few had a back room full of random item bargains.
Charity Shops. They get LOTS of stuff sent in by people who treat the store as a garbage disposal. Effectively, once the charity shops have the gear in their possession, the property belongs to them. I would ask them if they have any items that they deem unfit for the shelves that you could consider. Donate a small fee and you will have mountains of stuff to use.
Personally I have used my Dad's garage as a place to find stuff. Any retired man will have a mountain of useful nic-nacs stored in a cupboard or garage; I've managed to get hold of some great items and it all helps clear out the stuff early before they croak it! Seriously, some great stuff has been gathered from the good man.