He did say it has pins on the processor, so I'd assume it's a Socket 478 model. Finding a motherboard that old is going to be difficult, all of them will be used, there aren't likely to be any in new or like new condition. I can only suggest trying some place like eBay to try and find one.
Do keep in mind that you will also need all sorts of other obsolete hardware to complete such a build, like SDRAM or DDR RAM, an AGP video card, and odds are your Hard disks and optical drive will probably be IDE rather than SATA.
Really, you're better off with a much newer system. Even the cheapest new system will outperform that old Pentium 4, and will be compatible with current hardware. With this Pentium 4 processor you will need to hunt down all sorts of obsolescent equipment to get a working system out of it.