Buying aftermarket will not work in your car because no one makes an aftermarket manifold for it. On your car the cat is part of the manifold so if you replace it with aftermarket you will not have a cat, which is illegal. You need to find a used part, weld the crack (they have been known to crack again after welding) or buy a new OEM manifold from Honda. Its $216 plus shipping from the link below. You need part number 3.
http://www.hondaautomotiveparts.com/...NIFOLD+%281%29
If you are worried about buying a new manifold just so it might crack again you have another option. I did this when my HX manifold cracked. I bought a civic EX manifold (regular style with downpipe, no cat) and bought a direct fit cat from Magnaflow for the civic EX. The manifold bolts right up to the existing exhaust but you dont have a cat, so I needed to take it to a shop and have them cut my midpipe to fit the cat in. Essentially I converted my exhaust to a regular civic exhaust with header, downpipe, cat, then midpipe. I had to extend my o2 sensor wires as well. This cost me about $200 but it will cost you more because the direct fit cat for CA cars is $150 more than mine cost.