How can i find a 450+watt power supply for my old dell?

im purchasing a hd3850 agp graphics card for my old dell. it recommends a 450Watt or greater power supply with 30Amps on 12 volt with 2x4 power connector. my mobo needs a 20+4 pin connector and i need at least 6 4pin molex connectors. how can i find the right internal power supply for my needs? can someone recommend a power supply that fits this description?