You can go on eBay, Amazon, or a local thrift store or pc shop that has some scraptops or lose cpus for sale. You need a pentium 4 mobile edition and will need to look up what models it supports and yes you can upgrade it. For the nubs that didn't know that intel had made mobile pentium 4 yes they did and for a while they did sell well due to the brand. Some did run at rather decent wattage but the clocks were not so great.
Load up CPU-Z and then tell us what core and what model that your celeron is. By the way you can use your old cpu in a desktop socket 478 board if you wanted a cheap FreeNas box that didn't use to much power.