how do i buy another 780i bios chip?

First off does the board have a removable BIOS chip? If it does you can buy one cheap like $25 but many of the new boards have the chip as part of the board where it can't be removed.

If its removable you can get a new one at www.badflash.com

 
A quicik look shows this board with a removable BIOS chip so you should have no problems getting a new one. As I said earlier you can buy a new chip from Badflash and get it with any BIOS revision you want installed. Then you just carefully remove the old one and install the new chip clear CMOS and reset just like as if you had flashed the BIOS. Now also they will reflash your chip if you send it to them and thats even cheaper then buying a new one. They also ship world wide.
 
why get them to flash the old one when you can do it your self by hot flashing it
 
if you know some one with an 8 meg (which yours probably is-please check) bios board and a removable bios chip then what you do is pop out there bios chip with the pc off, then push their bios chip back into the slot just enough for it to boot the pc, once the pc reaches windows use a wooden tooth pick so that you do not short anything and flick out the bios chip, put your bios chip in the running pc and use the right flashing program for your chip to flash your chip back, then turn off the pc and put back the bios chip and try your bios chip in your pc, i have done this once b4 so i know it works....good luck you will find a lot of imformation on this on the net search for HOT FLASHING.......!
 
you can remove BIOS chips with a special tool, I did stuble upon it once, but its a pain to fit I perosme. wouldn't it be best to RMA the baord?