Gart Sports carries them. They are about $1 per pair for the hands and $1.50 for feet. But I suggest you depend on clothing more then on the hand warmers. Get a pair of wool socks, preferably knicker socks, for your feet. Also get a pair of insulated insoles for your shoes (I use clipless all year long) and a pair of neoprene booties for covering your shoes. Some people like wearing Shimano sandals and warm socks covered by a wind or water proof sock. Whatever shoe you use, get it a size larger or 2 than you would normally wear. This improves circulation and keeps your feet warmer.

For gloves, I use a brand I get from Gart's (I really don't shop there much but sometimes you just have to) called Serius. I don't have problems with my hands getting cold (usually just the opposite) and these gloves are thin, windproof and work well for me. YMMV. I also use the Serius Ear warmer under my helmet.