Actually, I thought Kon's original post was clear, unusual, but clear. To build a PC around a used motherboard is a real stretch. If it's for a learning experience then it's a good project. If it's to end up with a good PC then there are much easier ways to go. By the time you buy a case, power supply, CD or DVD burner, memory, etc. you could be spending as much as a new faster entry level PC would cost.
If there is a local computer shop that works with you that would be your best way to find a case that works. Dell is not known for using standardized parts. I've worked with a lot of Dells in a work environment, and they are generally good machines, but not necessarily build for interchangeability or upgrading.
There are plenty of online sources (Newegg, Amazon, etc.) that sell cases, but there are a lot of minor, sometimes major, problems that can occur when screws, outputs, etc. don't line up.
