Once source could be residential remodeling companies, which typically do a lot of demo when they renovate. You'd have to either call around or stop at job sites when you see them. You're looking for old floor or ceiling joists, or rafters, which usually measure 2x6-12. When cutting, you need to be very careful about embedded nails. Wear safety glasses.
Alternatively, a municipal dump or recycling center can be a source of all kinds of awesome things. Not related, but I got a 15' x 42" vinyl pool last week, in perfect shape. At 4500 gallons, it would be a great reservoir for collecting roof water. But I don't own here so I'll have to either sell it or bring it back.
Here in NY, my entire garden is raised bed on ledge rock. When I planted two apple trees nearby, the amount of rocks I had to dig out was enormous. But they were pretty rectangular and of good shape, so I'm slowly using them to edge the beds I carve out. But really, you don't need to edge the beds at all. Just slope the soil at the sides a bit, and mulch. Hard edges are nice, but not necessary.