If you get a universal cable, it will have two different types of end blocks, one style for road brakes and another style for upright bars. Simply cut off the end you don't need.

Cables and casing can be had as cheaply as $6 at Walmart, however I'd suggest using better cables unless you are on a very strict budget. braking feel and performance will be much better. I usually buy something like this for my own bikes: Aztec cable & housing set at Nashbar

You may want to get a cable cutter as well. I've used wire cutters long ago, but you really are better off investing in the right tool. This one is $15 and works decently, though there are better ones that are worth the money. http://www.nashbar.com/bikes/Product...59_-1___202578