1) How often are these eggs found?
-Very rarely- generally people can go days and days using all their gather turns by scavenging and still not get an egg. You can also get them via the Coliseum as random drops, but this is even more rare than scavenging!
2) Do you have to meet specific requirements for finding them?
-Nope! Though the element of the egg you get will depend on what area you're scavenging in (plague, ice, etc).
3) Are all the eggs on the AH foundling eggs from scavenging or? (

) I suspect not since some were VERY expensive.
-You can only get them via scavenging and coli, so yup, they're all from that. Some are more popular than others, though, because of the eye colors. So ice eggs might be more expensive than earth- or the other way around, if more ice eggs are on AH!
4) Is it worth it to hunt for parentless eggs or just buying and egg and hatch it and see what comes out???
-Generally, I'd say look for people selling parentless dragons, or advertise that you're looking to buy them. Hatching eggs can sometimes give you stunning results that sell for a ton, but most often they're pretty ugly and need to be exalted- and with eggs at roughly 30k a pop, that's not always a great investment.