I'm not sure what capacity you need. They are probably very small capacity (a typical automotive condenser is a fraction of a uF) but will need to have a very high voltage rating because the EMF spike induced on the primary will be much higher than the nominal supply voltage. Also note that they need to be bipolar since the coil primary will oscillate when the points open. I suggest you just take a trip to your local car parts outlet and ask for an ignition condenser and hope that it has a high enough voltage rating for your fence - it will probably be OK. It would also be worth checking what's in the old driver module since it's possible that the capacitor is still usable (and it's also possible you could fix the whole thing just by replacing a blown transistor or replacing a fuse).
There's nothing wrong with doing things for the sake of it if that's what you want, but IMO the best projects to learn how to use an Arduino are ones for which an Arduino is a sensible solution - and this isn't one.