Newegg has a few (which they incorrectly describe as Socket 478), though you'll need to make sure whatever board you have can support Dothans. And they're out of stock right now anyway. Intel never made much effort to make these available as retail parts, so you may have trouble finding a good, cheap supplier.
No, you cannot use Socket 478 processors. You also cannot use Core Solos or Core Duos if your board was made for Pentium M (and vice versa).