If you can't find new stock of the CPU you want from suppliers, try ebay. I've never failed yet in finding the CPU I want there (although it can take a while!) - and I've not got a dud yet.
Agree with others that you should go for a better CPU than a (slightly) upgraded Celeron.
Check
CPU-World: Microprocessor news, benchmarks, information and pictures and
CPU-Upgrade: motherboard upgrade and processor upgrade resources as well as the Asus site for all the compatability information you need. Asus don't seem to update the CPU compatability list beyond a certain time after the board is retired.
XP is going to end of support in mid 2014.
SD