I found 1 Sedna card on Amazon.
LinkYou can also buy them directly from Sedna's shop.
LinkBear in mind that the card may not initialise on the XE. This is the case with anything above HD6000 cards on the Sam440 and is said to be down to Uboot. Since the XE's Uboot is older it's likely to have the same limitation unless something was different in the Sam440 version.
Do you have access the RadeonHD driver or do you just want to try and see if you get a display first? It may be worth asking A-EON if they can supply you with the limited demo driver included with FE for Sam460 for testing purposes.
I had an X1550 (PCI version) on my XE back in the days of the beta driver. It's been too long a time to remember specific tips but it's worth having 2 monitors handy while you're experimenting.
Remember that if you can't get this card to work that you can still use the adapter for older HD series cards. I know that the XE is capable of running at least HD4000 series cards.