i2s is digital so you need to build an ADC i2s circuit, for example using TI's PCM1802 chip or similar. That might be a project on itself.
And unfortunately you'll find that i2s for audio input is not very well supported on PIs, etc. and may require some kernel hacking to get it to work reliably.
So depending on your expectations, you might be better off just buying a $5 USB sound card and use that for input.
E.g.,: