AFAIK, eBay has
been the place. I've only once
had to buy one, and thankfully it was a socketed ROM.
The amazon link you provided implies you're in the US; so, eBay's probably an option. If your machine was 'commodity' and older, the whole dang mainboard might be an option.
(Either replacing it wholecloth, or kit-bashing over the SPI Flash ROM.)
edit:
Not to reinforce the whole
free internet tech support *thing* going on...
But, how did this situation precipitate?
I'm guessing you had a bad software flash, potentially of a self-modded or '
found online, somewhere' modified firmware?
Before you took it apart, did you investigate if HP had a 'blind recovery' method for bad flashes? It's not uncommon for HP, etc. laptops to have "Technical Manuals" available, or at least some HP Forum response about how to "Recover From a Bad BIOS Flash"?
The 'industry standard' generally (if so-equipped) is having the uncompressed firmware file sitting alone in the root dir of a FAT32-formatted flash drive.
Some units need the file renamed
Some units required a specific input sequence from the Keyboard and/or 'hot key bar' at power-on, while the prepared drive was inserted.
Even, some surprisingly-new mobos and laptops require a Factory-Made <4GiB USB Flash Driver