Did you mean a Pentium M with a FSB 400 MT/s? There is no such Pentium M i can find thats 400MHz.
In any case, intel has stop making those cpus. Meaning your going to have to look around on places like Ebay for a price.
http://computers.shop.ebay.com/CPUs-/164/i.html?CPU%2520Type=Pentium%2520M&CPU%2520Socket%2520Type=Socket%2520479&_nkw=Pentium%20m&_catref=1&_dmpt=CPUs&_fln=1&_ssov=1&_trksid=p3286.c0.m282