Build three and sell one and recoup your material cost entirely! Gees I am brilliant this morning

I've seen racks of 2"x4" cut to 2' and 4' lengths in both Lowes and Home Depot...

Woodcraft.com, rockler.com, hobby lobby all have what you are looking for.

All good advice, thank you team

(I forgot to mention, I'm in a golf cart so need manageable sizes to avoid attracting Smokey's attention)

I just sided my house with LP Smart Siding, which I think would be perfect. I have leftovers that I'll share. Sent you a PM.

You will get rats with birds
 

You can get scrape wood from them to build your bird feeders, but your neighbors will hate you because feeders draw rats here and other unwanted critters. Put in a bird bath or hummingbird feeder.

yes, our neighbor five doors down has a feeder spilling seed onto the ground and she has raccoons, rats, mice and ducks vacuuming up what the birds scatter

I'm looking to build a couple of replacement bluebird houses to replace the two that are falling apart

they have lots of wood, I assume if you are going to build a bird box you have a saw? They can cut for you but most wood boxes need lots of cuts so a saw is required. But you can buy all kinds of wood at lowes or home depot

Home Depot gave me some cut off's for free

Lowe’s will cut lumber to your specifications, no charge. Ace hardware has small finish grade boards too.

"Two that are falling apart." Choice of materials. proper construction are the difference between something that will last and something that won't. Typical construction lumber is PINE and it does not last long exposed to the weather. Pressure treat is probably not good for the birds. Cedar is a good choice. Hardwood? The box stores sell planed lumber. If, you choose that route the COST will be shocking. Bird houses? Should you bill them for materials and charge them a bond plus a mortgage they will advise you they are on a fixed income of zero.

Final update: found a nice piece of 2x4 planed (for the upright) that was around 3' long with a fractured break at one end. Took it to be priced. Scanned at $14 for the original and the lumber-man told the cashier to "put in 25". I assumed that meant 25% of the scan, or $3.50, which sounded reasonable. It actually meant 25c, which didn't seem too bad, either :)

Paid a lot more for the box-parts as there was nothing suitable in the off-cuts trolley, but ended up with two smart-looking bluebird boxes that I'm sure will attract a long line of prospective renters.

Here in UK I have made a lot of bird nesting boxes, hedgehog hibernation boxes, and compost bins with recycled pallet timber.
The leftovers are kindling.
All good stuff and free.

Arctic Fox, waiting for pictures!

Put the final coat of paint on this evening - wanted them to match the house so they'd feel at home - so just waiting for two families to start renting them