May be no one has stocked it yet. What you can do is connect four 24LC256 on same I2C bus giving different ID to each IC through its A0, A1 and A2 pins. This way also you'll get 128kB EEPROM.
For example,
Like kinjalgp, I also don't use PicBASIC, but to address different chips just requires changes to the address part of the I2C code. The larger EEPROM's use 2 byte addressing (whereas smaller chips only use one), it's all explained in the EEPROM datasheets.
You may also want to look at the Atmel serial DataFlash series. The interface is SPI compatible rather than I2C. Since it uses flash memory, the densities are considerably higher (1Mb - 128 Mb) than you'll find in standard EEPROMs.
Sorry, don't know the answer to that one, I haven't used PicBasic. SPI is a simpler interface to implement from scratch, it's a non-issue if your micro has an I2C interface built-in.