I recently had to bottle feed a Frenchie puppy and found that my mini  crock pot did a great job of warming the bottles.  I would leave it on all  day so it was ready at any time .  I used a digital meat thermometer set at  100 degrees .  Worked great- no guessing.  I also used a mini bottle.  The  kind you give babies medicine in.  Regular size nipple but only holds 15  mls of formula so you know exactly how much the puppy is getting.  Less  wasted formula.
photo credit: Renee V