I'd encourage someone to verify/deny this but:
Since the voltage specifications match and pinouts correspond, between the Atmega8 and Atmega328, I think you should be fine with either one. Select the right board from the Tools > Board > menu in the Arduino software.
Although... you'd have to purchase the chip WITH the bootloader in it already (alternatively, you could buy it and then burn the bootloader to teh chip using an additional Arduino, or other ISP/programmer).
When you say you can't find anything except Atmega8, do you mean logistically/geographically you can't find something convenient, or are you saying you can't locate it at any store online? (because there are many websites that sell Atmega328, with and without the bootloader)