Missing from the question is what the intended uses of the materials are.

The community gardening folks in my area have pretty close ties with the county folks who are trying to incorporate us. Wouldn't seem like to much of a stretch for our fellas to come up with where folks are getting building permits.

Something like "Bob down the street has applied for a permit to rebuild his xxx, bet Bob is going to have old materials he's getting rid of"

For sources check the free section your local Craigslist, or Freecycle. You might also put an ad on a free bulletin board.

Your concerns about chemicals are valid, however I think most metals are quite safe. As you said railway ties are a no-no in any garden.

There is a forum here on GW where creative people do this all the time. Link at bottom of post.

Many people use metal headboards and footboards as fences and trellises. Old windows are used to make greenhouses and gazebos.

Not sure what you could use for raised beds but if there is someone who knows how to weld many things can be fabricated from metal.

Here is a link that might be useful: Garden Junk forum

But is the overall carbon footprint really being served, if you have to drive all over town trying to collect these re-used materials?

The legs off of old folding lawn chairs can be used for garden tunnels.

Layers of newspaper with mulch on top will keep down the weeds between the raised beds. For mulch you could use sawdust from somewhere they do woodworking or the long red pine needles or check around for something else that's available.

You could use reclaimed lumber from building sites for the raised beds. The trouble with reclaimed lumber is that it may rot fast but if you painted it or lined the beds with styrofoam that would help. My friend works at a lab and we got a bunch of square styrofoam that comes in 2 pieces that their vials come in to protect them.

Old hoses that leak can be turned into soaker hoses by punching more holes in them.

I don't know how much room you have but tires filled with soil make good tomato beds for 1 plant or a plot of lettuce.

Above all learn to compost all your garden scraps.

Recycled concrete (old walkways, driveways, etc.) from ongoing construction/remodelling projects is a great reusable material. Makes fine raised beds or low stacked walls or large chucks can be used as pavers for new walkways or patios. Craig's list or freecycle are great sources for this type of stuff but check out salvage yards as well and keep an eye on constuction sites.