try sending them an email. According to Asus they should be keeping stock on obsolete parts at this site. If no buy button should mean they are out, but they should be able to tell you when they will get more. This wass all the info I could get out of Asus. Sorry I could not help more.
Thank you for contacting ASUS Estore. However that item is currently out of stock and at this time we do not have an ETA of when it will become available. I do apologize for the inconvenience this maybe causing you. PLease check back with us within a few weeks.
You might not (probably don't) need a 650W ... Most likely 350W-520W will be more than fine for whatever you have in the machine.
Measure the dimensions of the bad power supply and just look up the different Supermicro power supplies and find one that is around same dimensions (or smaller).
Or you could also buy a new chassis for <$249 depending on the dimensions of the motherboard.
I have 2 Quad-Core Xeon E5320, 4x4GB FB-DIMM, 4xSATA on PCI-Express RAID card. According to the Antec Power Supply calculator, Im in need of 414watts minimum. Running a 520W would be running the PSU at 80% of his capacity. Too much for me. If I'm going after markter, I'm targeting 700W+ with a good effiency.