Which chip technology? Parallel/LPC/FWH/SPI? There are multiple solutions for all chip technologies out there, some of them easy to build, others require complicated PCBs and expensive parts.
Easiest would be to pick an existing opensource project and use it (or extend it if you need something special).
- For SPI, the RayeR (Martin Rehak) stuff is neat and I heard it works fine. You can either use RayeR's SPIPGM or the patch for flashrom(.org) which is floating around.
- For LPC (and to some extent, FWH) you can use the Milksop or LPC^2 projects. No idea how much code/schematics are still available on the net. There is some preliminary patch for flashrom which would enable you to use that hardware. No idea where you can find that one, though. Maybe ask.
- For Parallel, you can either abuse old network cards for flashing, or you can use some parport/AVR thing (google for "serprog flashrom") or build something yourself.
Neither of the solutions above is sold anywhere, it's all real do-it-yourself stuff.