Sometimes they are steel. I wonder if you could buy them from a big box store? A kit perhaps waiting for a handy person to make a headboard? Anywhere that sells mattresses sells metal frames..fairly cheap.
DH used 2x8's but 2x6's will work. You need a 2x2 kleet on the inside edge to support the 3/4" plywood. Good bed rail fasteners are out there. Don't use the cheep ones.

You can get queen size bed rails at Amazon.com and buy.com.

I did the same thing, purchased the HB and FB of an antique and no way could I find rails. I did find them on-line but after careful discussion, the hubby decided he could make them and just buy the metal hooks that go on the end of the rails that latch on to the head and foot boards. So we bought poplar 1/4", painted them attached the hardware and that was it. I also purchased an extra brace for the rails to hold the mattress and box springs so nothing would go crashing down. You can buy both wood and metal on line though. Just do some google searches.