Hi,
I use http://bios-repair.co.uk/ it sends chip very cheaply (postal one pound to CZ) good price if you need single chips
Also I buy some chips from farnell.com (more you buy more cheap), to avoid postal fees I buy it through local czech company.
They had only one DIP8 SPI flash chip (that AMIC) so in general it is not so easy to get such chips. Mauser, digikey might have more.
I think others can write their sources ;)
Thanks Rudolf