First let me point out to you that a 0.47uF capacitor at 13KV holds about 40 Joules of energy. That much energy at such a high voltage is extremely dangerous, and can easily kill people. Hell, any circuit with 13KVDC is deadly dangerous. But you should exercise extreme caution, and I recommend starting off with a much smaller capacitor value (like 10s of nF).
Anyways you'll usually want to look at industrial surplus dealers. You can sometimes find amazing stuff for very low prices. Ebay is also an option.
Microwave oven capacitors are generally ~1.0uF and are rated for >2KVrms, and are pretty easy to find (both in a junkyard, and on the market). A bank of those would probably work fine (with balancing resistors as FvM suggests).
And again, please be careful.