Boiled wool is not a term you hear too much anymore. More often you will hear it called "felting". Whatever you call it, the result is a thick, durable, waterproof fabric that makes the perfect winter mitten for pennies. The only time I knit mittens anymore is for the fun of it.
At our local thrift shop Wednesdays are half price days. That means that I can usually find a 100% wool sweater for about $1.50. If it is a large I can make several pairs of mittens out of it. Merino and lambs wool make for a softer end product than just plain wool, so if you have a choice go for the nice stuff (cashmere, anyone?).
The sweater choices are usually numerous. After all, no matter how well you take care of a sweater eventually it will wear out.