Try the larger metal recycling places for your pieces. Stop by weekly if you need something specific. Eventually you will get to know someone there and they will keep and eye out for it and set it aside for you. Tell them what you are building and what parts are needed. You might be surprised what they might set aside.
One of the places that I deal with set aside 10-NOOK EZM-3015 bearing blocks(brand new). They are double angular contact bearings in a block assembly. I got them all for $15.00, they retail for $300.00 each. What a deal.
As for "diving", it can be helpfull if you get written permission from the company. Just explain to them what you are doing with the materials. The permission thing has saved my butt several times at 10 pm. with spotlights in my eyes. Good luck!
If it's not nailed down, it's mine.
If I can pry it loose, it's not nailed down.