Hi, Definitely try the kitten formula. Wal mart sells it in powder or liquiid formula over in the pet section. They should also have the small pet bottles with nipples. My recomendation-have more than 1 bottle. I cared for baby squirrels before, and small critters can tear through the nipples quickly!
Also, don't squeeze the formula down-just make sure your hole in nipple is big enough for milk to come out, but not pour out. Worse case scenerio-use a shot syringe (from doctor or vet). The tip will hold up much longer and baby bunny will adjust to learning to "nurse" through the harder tip while you gently squeeze.
And yes, you will need a warm heat source but not hot. YOu may want to consider either a small heating pad on LOW-but bunny may start chewing soon. You can also do the hot water bottle, and you may want to use one of the clip on silver reflector lamps with a 60 watt bulb. But whichever you chose-make sure baby can move away from the heat source if it gets too hot. Keep it from drafts.
Any questions just email!