Homebrew, but here's my part list:
Product: Plastic Battery Holder 1-cell ø20mm PC 120591-1
http://www.dipmicro.com/store/BH120591-1
Product: Lithium Battery CR2032 Ф20mm 225mAh 3V CR2032
http://www.dipmicro.com/store/BAT-CR2032
Product: 4.7k Ohm 1/4W Resistor ±1% 4k7 472
http://www.dipmicro.com/store/R3F47-4
Product: 32.768kHz Crystal Cylinder
http://www.dipmicro.com/store/XC4-32768
Product: DS1307 Dallas Maxim 64 x 8 Serial Real-Time Clock DIP8
http://www.dipmicro.com/store/DS1307N
And I wired it up based on the 'typical operating circuit', except without the scl being pulled to 5.5V.
It's also all wired up on a breadboard.