Online stores - I should look into the vendors you suggested to me before. Just need more hours in a day.
Must say I haven't experimented with long distance I2C, but perhaps using I2C bus extenders might be an idea. The datasheet of the P82B715 I2C bus extender talks of communication over a twisted pair wire with length of 50m/160ft. It seems quite easy to work with and the price (at Mouser, haven't looked further) also seems reasonable
644/1284 is easy to breadboard, same xtal/caps/reset resistor as the '328.