Hands: On the coldest days I use these Burton mittens with the heat packs. The mittens have a little fleece lining and this handy zip pocket on the outside. I just slip in the packet right there. I have also dropped them in a windproof light-weight mitten and wear another glove (if the temperature requires it) inside. I can bike along squeezing/touching this little heat pack and warming the fingers. I have used them with some fingered gloves too, by inserting them down in the glove to the palm, or on the back of my hand. The heat will still radiate to the fingers. With it next to the palm, it still isn't too much in the way when you have the hand on the handlebars of the bicycle.
I can buy them at my local Gander Mountain store. But this season since I have been biking every day in the cold weather I have purchased them online in a 40 pairs count box. Cheaper that way. I have to check several websites because the prices vary. I like to go with who ever has the cheapest (or free!) shipping at the time. There are several brand names of these instant charcoal activated heat things: Toastie Toes, Heat Factory, Hot Hands to name a few.
Now I have figured out how to use the hand and the toe warmers TWICE. As soon as I finish my bike ride (or skiing, or snow blowing) I put them in a zip-lock baggie and zip it closed to cut off any air/oxygen. They are reusable the next time. So this cuts the cost down to half the price.