Does Home Depot have a scrap metal pile where I can get some for free? Anywhere else?

Home Depot has no such thing. They are a retail outlet, if you want something from them, you're expected to pay for it.

Check out junkyards in your area.

SOME locations have 'remnants' at a reduced price, but for liability reasons, they don't give anything away.

call plumbers

They may, depending on if they have in house service for equipment. Here is the thing about any of those scrap piles, usually in a container like a dumpster. The business sells that in an effort to keep their cost low. If you talk to someone, and need a little bit to complete a project, they may let you have a little. If you want to raid their pile to sell it yourself, no, and taking it without permission is stealing, and you can be charged accordingly if you do it