Repair or Upgrade Centrepull Brakes?
The first thing to note is that v brakes need their own levers (long pull), as they don't use the same cable pull as standard cantilever brakes, so that may add an extra expense (unless you have one of the few levers which are switchable between V-brakes and regular brakes). Thus, if you want to switch to V-brakes, you need either {a V-brake compatible lever (long pull) and a V-brake} or {a regular lever (short pull) and a travel agent (essentially a pulley) and a V-brake}. The option of a V-brake compatible lever and V-brake is typically the better and cheaper one if you want to go V-brake (unless you need to use an existing lever, such as Shimano STI levers with a V-brake, in which case you need to take the other option). Note that if you have an integrated brake and shift lever, going to V-brakes would also mean buying a new shifter as well. (*)

As for what to buy, we don't do product rec on this site. However, for drop bars, the Tektro RL-520 brake lever is pretty much the only model (aside from a rebadged Cane Creek) out that I know for V-brakes. For flat bars, the options are pretty much all there - Avid, Shimano, Tektro, etc. all make levers marked for V-brakes (or equivalently, non-road mechanical disc brakes) - pick one which feels nice. Similarly, those 3 companies also make a wide variety of V-brakes. My personal setup is Shimano Alivio brake levers with Avid Single Digit 5/7 V-brakes on a bike with flat bars.

Trying new pads (esp. if the old ones are worn/dry) may help. Adjusting things like the mechanical advantage may also be necessary - this link may be of use, and new cables may be in order for an old bike as well. Certainly, a new set of pads or even a new cantilever will be cheaper than switching to V-brakes (and may be perfectly adequate), but you'll likely get better stopping power easier with a V-brake setup. 

(*) There exist mini-V brakes which work directly with short pull levers, like Paul's Mini Moto. These are generally not cheap though.