Don't go out looking for a 1.5 or you will end up paying over the odds.
Most PSP's can now be downgraded to a 1.5 anyway, so don't worry about the firmware revision.
you should be aware that newer PSP's come with a TA-082 motherboard which cannot be downgraded, but even this can utilise an "e-loader" which will allow you to run emulated games (snes, megadrive, etc) on your psp without needing a 1.50 firmware.