1. eBay may be the only source at present apart from pot luck at a computer fair.
2. Which replacement? - depends entirely on your motherboard.
Its manual (if you still have it) will define the CPUs that it can take.
2a. If you replace the motherboard and CPU the field is wide open to your choice......
..... until you think about old RAM specifications which after 5 years will probably be incompatible with a new mobo.