er... be very careful about using I2c over range. It really is a bus better (there's that word again) used between chips on a board rather than wired between boards. RS232, CAN, even TTL serial or parallel are better options for that. The reason being the resistance and capacitive limits of I2C are very small.
Finding a good I2C chip is a great idea because you don't need many ports on the processor (it is a 2-wire bus that allows multiple components to connect together and to two pins on the cpu). I still would recommend running a wiring harness to each of the nodes of the battery net and keeping your ADC's all on one board.
Digi-Key Part Search
The link above will get you to the parametric search on the digikey website under the ADC section. You select your bus (I2C, serial, whatever) then select your voltage range, temperature range, your chip package (dirt-sized SMT or big honkin DIP packages) and all the other necesities. It will allow you to find multi-channel adc's and (literally) all the other options out there.
Another caveat with I2C components... some of them (including the I2C ADC that is on a board that I am currently working with) have a fixed adress, so you can't put multiples of the same chip on the same bus or they won't know which one is being adressed. If you can't find a way around this, you will have to use an analog multiplexer to feed into a single adc.
ps - what's an attiny device?