Sounds like you need an electrolytic capacitor, all of which are polar, as Jallen infers. Very good ones include very large ones made by Mundorf (which may in fact be nonpolar; I forget) and more reasonable size polar ones made by Panasonic (TS-ED series) and Nichicon (FX series). The Mundorf are by far the most expensive but worth it if you have the room for them on your chassis. I think you can buy them from Parts Connexion in Canada. The Panasonic and Nichicon are relatively tiny and dirt cheap and can be purchased from Digikey or Mouser in the USA.