Where can I get a 450 watt power supply?

[ArchAngelAlien]

I need a 400+ watt power supply for my Dell Inspiron 546 ST Slim model case. This is what the case looks like:

http://www.cedarpc.com/product/9580/Dell-Inspiron-546s-Slim---9580/

It's very slim and small and regular psu's won't fit inside it, I need to replace the Geforce 210 512 64 bit video card inside it with this:

http://www.powercolor.com/us/products_features.asp?id=255#Specification

The upgrade is for Cataclysm but the video card requires around 400+ PSU and a 6 pin connector. My current PSU is a 250 watt with a 12V rating of 17A. Can I use my PSU and 3 way split the power from my dvd drive? If not where can I find a PSU capable of fitting in my case and handling this video card?

I cannot swap out the case components to a different case. it's a family PC. I don't want to downgrade to a different video card, there has to be some way to make this work and I came to where the most elite computer people can help me out.

I can run the case without the side panel attached with a air conditioner pointed at it for all I care for overheating issues, I need this video card for WoW Cataclysm, after getting accepted into the beta, I cannot run the new water affects without a fps over 10 on ultra...

 

[ArchAngelAlien]

Okay um, my motherboard connector for power is a 4 pinn, I think..? This will power my motherboard and that video card no problems?