The ATmega328 and ATmega328P will also work (and have twice the memory as the ATmega168).
Can you find AT90S chips (made by atmel)? I'm not sure they are pin compatible, but they are ATMEL chips and have the AVR GCC support for them. If you're looking for the bare chips, chances are you can program them, so it doesn't quite matter if they are ATmega or not. @Senso, AVR chips aren't easily available everywhere... if it hadn't been for the Arduino, you'd probably still be "robbed" every time you wanted an AVR in Portugal.
A couple people from the Philippines have managed to build Arduino systems. Maybe they can point you at their sources. Search the forums for "Philippines."
I can assume any chip made by atmel can be used as a replacement for the atmega8 and 168?
Well, no. Almost any "8bit AVR" will be supported by the underlying tools that are used by the Arduino IDE, but only a few are immediately compatible with the Arduino SW. Given the nature of your questions, you should probably put your efforts into finding a mega8, 168, or 328, rather than trying to figure out how to make some other chip work...
http://www.ebay.ph/No local sources of Arduino but plenty of choices for places that ship to the Philippines.