To Tobar, about "old enough to be found in a junkyard":
1) So what about it? Seems like these droids were around for a long time.
2)As it's known, Anakin found parts, not a whole broken droid at once. So lets say that someone had his 3PO fixed, and trows out a part. Then another person does the same, but with different part. Watto had huge junk yard, so it probably had a lot of stuff. It's also believed that Anakin might have... ahem... "borrowed" some parts from actual shop (at the background in Watto's store you can see a protocol droid with missing parts).